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Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents
Sleep Medicine xxx (2014) xxx–xxx
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Original Article
Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents T.H. Stea ⇑, T. Knutsen, M.K. Torstveit University of Agder, Faculty of Sport and Health Sciences, Kristiansand, Norway
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 10 January 2014 Accepted 11 January 2014 Available online xxxx Keywords: Adolescents Sleep duration Overweight Obesity Health-risk behavior Academic achievement
a b s t r a c t Objective: To investigate the prevalence of short time in bed (<8 h/day) and to examine the association between time in bed, overweight/obesity, health-risk behaviors and academic achievement in adolescents. Methods: This study included a sample of adolescents (n = 2432) aged 15–17 years in the southern part of Norway (participation rate, 98.7%). A self-report questionnaire was used to assess time in bed, body mass index, dietary habits, physical activity habits, sedentary behavior, smoking and snuffing habits, and academic achievement. Results: A total of 32.3% of the students reported short time in bed (<8 h/day) on an average school night. Several health-risk behaviors were associated with short sleep duration, including not being physically active for P60 min for P5 days/week (adjusted odds ratio, 1.33; 95% confidence interval, 1.05–1.68); using television/computer >2 h/day (1.63; 1.23–2.17); being a current smoker (2.46; 1.80–3.35) or snuffer (2.11; 1.57–2.85); having an irregular meal pattern (1.33; 1.05–1.68); intake of sweets/candy P4 times/week (0.51; 0.32–0.83); and poor academic achievement (1.62; 1.26–2.09). All odds ratios were adjusted for sex, age and parental education. Conclusions: In Norwegian adolescents, short time in bed is associated with several health-risk behaviors and poor academic achievement. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Adolescents’ sleep duration has decreased over the past century, and research has also indicated that the pattern of decreased sleep duration increases with age [1,2]. Studies have estimated that most adolescents need 8–9 h of sleep per night [3,4], and the National Sleep Foundation, a US-based non-profit organization, has estimated that most adolescents need 9 h of sleep, and that <8 h of sleep on weeknights is characterized as insufficient sleep duration [3]. Although European guidelines for sleep duration are lacking, several studies have shown that short sleep duration (<8 h/day) is associated with several negative health consequences such as high blood pressure [5,6], insulin resistance and diabetes [7–9], decreased immune response [10,11] and mental health problems [12–18]. If left untreated, sleep disturbances at age 16 years have been shown to predict sleep disturbances in adulthood [19].
⇑ Corresponding author. Address: University of Agder, Faculty of Sport and Health Sciences, PO Box 422, 4604 Kristiansand, Norway. Tel./fax: +47 38142324. E-mail address: [email protected] (T.H. Stea).
Several cross-sectional studies have shown that short sleep duration is associated with high body mass index (BMI; kg/m2) and a prevalence of obesity in children and adolescents [20–22]. Some studies suggest, however, that these associations are apparent only in girls, not in boys [23,24]. Longitudinal studies have also reported conflicting results; some confirm an inverse association [25–27], whereas other studies have not detected any association between sleep duration and obesity [28,29]. Moreover, another longitudinal study reported that the association between sleep duration and BMI varied by sex and age, with stronger associations in boys than girls and in younger children compared with adolescents [30]. Previous studies have also shown inconsistent findings regarding the association between sleep duration and food intake, with a study reporting that one night of reduced sleep subsequently increased food intake among young healthy men [31]. However, a recent study reported that sleep restriction was associated with a small negative energy balance driven by an increased energy expenditure from prolonged wakefulness, and a concomitant decreased energy intake and motivation to eat in male teenagers [32]. Other studies have shown an association between short sleep duration and low meal frequency [33], high intakes of snacks [34]
http://dx.doi.org/10.1016/j.sleep.2014.01.019 1389-9457/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Stea TH et al. Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents. Sleep Med (2014), http://dx.doi.org/10.1016/j.sleep.2014.01.019
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and soft drinks [35,36], as well as low intakes of fruits and vegetables among children and adolescents [21]. Several studies have reported an association between sleep duration and screen time, whereas short sleep duration has been associated with using a computer P2–3 h per/day [36,37] and watching television >2 h per/day in adolescence [21]. In addition, several studies have shown that short sleep duration is associated with low levels of physical activity in high school students [21,36,37]. Furthermore, smoking cigarettes in adolescence has been associated with insufficient sleep on weekdays [36,38], inconsistent sleep patterns between weekdays and weekends [39,40], delayed sleep phase and other sleep disorders [41,42]. Few studies have focused on the relationship between the use of snuff and sleeping habits in adolescent years; however, a study targeting an adult population reported that the use of snuff has increased the odds of insufficient sleep compared with those not using these tobacco products [43]. Insufficient sleep duration may reduce the ability to concentrate and retain information [44], and shortened sleep duration and delayed sleep have been inversely associated with school performance [33,41,45]. A meta-analysis by Dewald et al. [46] also concluded that sleepiness and a low sleep quality were associated with a poor school performance. Nevertheless, other studies have reported no association between sleep duration and academic achievement in adolescents [38,47]. Because previous studies have shown inconsistent results, there is a need for additional research on sleep habits and associations with negative health consequences, health-risk behaviors and school performance. Thus, the main aims of the present study were to investigate the prevalence of short sleep duration (<8 h), and to examine possible associations between short sleep duration, overweight/obesity, several health-risk behaviors and academic achievement in Norwegian adolescents. We hypothesized that short time in bed is associated with overweight/obesity and the following health-risks behaviors: low physical activity level, high screen time, smoking and snuffing, an irregular meal pattern and high intake of unhealthy food and beverages, as well as poor academic achievement.
2. Methods 2.1. Study design and participants This cross-sectional study is part of a large, school-based cluster-randomized intervention study, ‘Active and Healthy Youth’, that promotes a healthy diet and activity patterns, and aims to prevent disordered eating among adolescents. The target group was 15–17-year-old high school students living in the southern part of Norway, and in agreement with school boards/school principals, a total of 17 out of 23 schools (73.9%) decided to participate in the present study. The main reason why the six schools did not want to participate was due to lack of time (five schools) and participation in similar public health projects (one school). A total of 2619 out of 2653 eligible students agreed to participate (98.7%) and the data collection was conducted in the period from October 2010 to February 2011. During the data analyses, a total of 187 students were excluded because they did not meet the age requirements for participation. Hence the data analyses in the present study were based on a total number of 2432 participants between the ages of 15 and 17 years. The Regional Committee for Medical Research Ethics approved the study protocol, and written consents were obtained from all students prior to participation in this study.
2.2. Measurement methods and procedures The participants were asked to complete a 15-page questionnaire that included questions about bedtime and wake time, body weight, height, exercise habits, screen time activity, smoking and snuff use, dietary intake, meal frequencies and paternal and maternal education level. In order to test the reliability of the questions used in the present study, the questionnaire was test–retested in a method study among 143 adolescents, aged 15–17 years. Test–retest reliability was measured by the intraclass coefficient (ICC). The questions included in this study demonstrated good test–retest reliability with a correlation coefficient ranging from 0.68 to 0.99. The students used 30 min answering the self-reporting questionnaire, with at least one member of the project team present to provide information about the project and answer possible questions. Sleep duration was assessed by asking: ‘When do you usually get out of bed on a school day?’ and ‘When do you usually go to bed on a school night?’ on a typical weekday. These items were used to compute the average total hours of daily time in bed, and responses were dichotomized into <8 h (short time in bed) and P8 h. The ICC (95% CI) was 0.83 (0.75–0.89) for bed time and 0.73 (0.61–0.82) for wake time. Self-reported weight and height were used to calculate BMI; categories were calculated using sex- and age-specific International Obesity Task Force (IOTF) cut-off points for defining overweight and obesity in children and adolescents aged 2–18 years [48], though, due to small numbers, the overweight and obese groups were collapsed into one group. The ICC (95% CI) was 0.98 (0.98–0.99) for weight and 0.99 (0.98–0.99) for height. Diet was assessed by a food frequency questionnaire of 13 items, including both food items and drinks. The questions had eight different response alternatives, ranging from never to more than once a day, and the response alternatives were further dichotomized into having a high or low intake of the selected food items and drinks. Having soft drinks, sweets and candy and a salty snack P4 times a week, respectively, was categorized as a high intake. The ICC (95% CI) was 0.85 (0.77–0.90) for soft drinks, 0.80 (0.70– 0.86) for sweets and candy and 0.75 (0.63–0.83) for salty snacks. Meal frequency was assessed by questions such as: ‘How often do you have breakfast each week?’, with the same asked for lunch, dinner and the evening meal. Response alternatives ranged from never or seldom to seven days a week, and in a statistical analysis the numbers of days reported eating breakfast, lunch, dinner and evening meals were then dichotomized into having an irregular meal pattern (omitted these main meals at least once a week) and having a regular meal pattern (intake of these main meals every day). These dichotomous variables were then combined to create a summary variable referred to as ‘irregular all four meals’, i.e. those skipping one of the main meals at least once a week versus those eating all meals every day. The ICC (95% CI) was 0.91 (0.86–0.94) for breakfast, 0.78 (0.68–0.85) for lunch, 0.68 (0.55– 0.79) for dinner and 0.89 (0.83–0.93) for evening meal. Physical activity level was estimated by asking: ‘How many hours per week do you spend on doing sports/physical activity in a way that makes you breathless or sweat?’ Response alternatives were: ‘0 h, 1–2 h, 3–4 h, 5–7 h, 8–10 h and 11 h or more.’ For the statistical analysis, not participating in 60 min of physical activity every day was characterized as health-risk behavior. Thus, the response alternatives were dichotomized into participating in 60 min of physical activity every day or more versus not engaging in recommended levels of physical activity (<60 min of physical activity every day). The ICC (95% CI) was 0.91 (0.87–0.93) for physical activity. Television viewing and computer use were enquired about as follows: ‘Excluding school hours on a regular weekday, how many
Please cite this article in press as: Stea TH et al. Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents. Sleep Med (2014), http://dx.doi.org/10.1016/j.sleep.2014.01.019
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hours do you watch TV or using PC/games?’ Response alternatives were: ‘I don’t watch TV or do gaming activity on a regular weekday, less than an hour a day, one hour a day, two hours a day, three hours a day, four hours a day or five hours or more a day.’ This variable was dichotomized into <2 h/day and P2 h/day, according to the American Academy of Pediatrics’ recommendation of screen time limits for children and adolescents [16]. The ICC was 0.81 (95% CI, 0.72–0.88) for television viewing and computer use. Smoking and snuffing habits were assessed by the question: ‘Do you smoke/use snuff?’ The response alternatives were: ‘Have never smoked/snuffed; have tried smoke/snuff, but not anymore, have smoked/snuffed regularly, but not anymore; smoking/snuffing, but not regularly and smoking/snuffing regularly and about . . . cigarettes/day.’ For the statistical analysis, those who reported smoking or snuffing occasionally or daily were classified as being a smoker/snuffer. The ICC (95% CI) was 0.95 (0.92–0.97) for smoking and 0.93 (0.90–0.96) for snuffing. The parental educational level was assessed with the question: ‘What level of education do your parents have?’ The question had four response alternatives: (i) elementary school, (ii) high school, (iii) college or university (63 years), and (iv) college or university (>3 years). These response alternatives were then dichotomized into lower and higher education levels (lower: no college or university education; higher: having attended college or university). The ICC (95% CI) was 0.83 (0.77–0.88) for maternal education and 0.80 (0.72–0.85) for paternal education. Achieved academic achievement was enquired about as follows: ‘In the last semester, what were your achieved grades in mathematics, English and Norwegian?’ A six-point grading scale (range, 0–6) was used to evaluate academic achievement, in which 6 was the best grade possible to obtain in Norwegian high schools. For statistical analyses, the mean grade was calculated and the results were dichotomized into poor (range, 0–3) and adequate (range, 4–6) academic achievement scores. The ICC (95% CI) was 0.82 (0.73–0.88) for achieved grades in mathematics, 0.83 (0.75– 0.89) for achieved grades in English, and 0.76 (0.65–0.84) for achieved grades in Norwegian.
Fig. 1. Distribution of sleep duration on school nights, by gender. The number of adolescents reporting short time in bed (<8 h) is presented by the bars representing 66 h and 7 h in bed, respectively.
calculate differences in overweight, physical activity, television/ computer use, smoking and snuffing habits, meal pattern and dietary intake between those reporting short time in bed (<8 h/ day) and adequate time in bed (P8 h/day). Multiple logistic regressions were used to explore the relationship between short sleep duration and the health-risk behaviors mentioned above, and the regression model was adjusted for age, sex, and parental education. Odds ratios were presented with 95% confidence intervals (CIs), and two-tailed P < 0.05 was considered statistically significant. 3. Results The mean time in bed on an average school night was 7.8 h (95% CI, 7.8–7.9) and appears as a bell-shaped curve for both sexes (Fig. 1). Approximately 32% of the students reported short time in bed (<8 h/day) on an average school night, whereas the number was significantly higher among boys (36%) than among girls (29%) (P < 0.001). Table 1 presents overweight/obesity, multiple health-risk behaviors and academic achievement relative to time in bed (<8 h/day vs. P8 h/day) adjusted for age, sex and parental education. The results did not show any association between time in bed and overweight/obesity, nor when analyzing the overweight group and the obese group separately. Those reporting short time in bed had higher odds of not participating in 60 min of daily physical activity (adjusted odds ratio [AOR], 1.33; 95% CI, 1.05–1.68),
2.3. Statistical analysis All statistical analyses were performed using SPSS version 19.0 (SPSS Inc., Chicago, IL, USA). In Table 1, the results are presented as a percentage of participants and v2-tests were performed to
Table 1 Prevalence and adjusted odds ratio (AOR) of overweight/obesity, selected health-risk behaviors and school grades by sleep duration.
Overweight/obeseb Did not participate in 60 min of physical activity every day TV/PC (>2 h/day) Current smokerc Current snufferc Irregular breakfast Irregular lunch Irregular dinner Irregular evening meal Irregular meal patternc (<4 meals per day) Regular soft drinks (P4 times a week) Sweet and candy (P4 times a week) Salty snack (P4 times a week) Low school gradesd
AORa (P8 h is the referent)
95% CI
p-value
16.3–20.4 55.9–60.9
1.22 1.33
0.91–1.65 1.05–1.68
0.191 0.018
73.1–77.3 10.7–13.9 10.9–14.2 31.0–35.7 44.8–49.8 23.1–27.5 59.9–64.4 75.1–79.4 26.7–34.5 10.6–16.4 10.6–16.4 26.9–42.9
1.63 2.46 2.11 2.24 1.57 1.87 1.44 1.79 0.79 0.51 0.82 0.80
1.23–2.17 1.80–3.35 1.57–2.85 1.77–2.84 1.25–1.97 1.46–2.40 1.13–1.85 1.32–2.43 0.52–1.18 0.32–0.83 0.46–1.46 0.71–0.91
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.004 <0.001 0.249 0.007 0.494 0.001
Time in bed <8 h
Time in bed P8 h
%
95% CI
%
95% CI
20.3 64.7
17.1–23.4 61.2–68.2
18.4 58.4
83.8 25.7 24.6 50.7 60.1 37.7 67.8 84.2 28.9 18.6 12.4 42.9
81.1–86.4 22.5–28.8 21.5–27.8 47.1–54.3 56.5–63.6 34.2–41.2 64.4–71.2 81.5–86.9 23.5–34.4 13.9–23.2 13.9–23.2 39.3–46.5
75.2 12.3 12.6 33.4 47.3 25.3 62.3 77.2 30.6 13.5 13.5 34.9
a
Adjusted for age, sex and parental education. Based on self-reported height and weight, body mass index (BMI = weight [kg]/height [m]2) P25, using sex and age specific International Obesity Task Force (IOTF) cut-off points. c Self-reported smoking/snuffing sometimes or daily. d Mean grade in mathematics, English and Norwegian language between 0 and 3 on a 6 point grading scale (0–6). b
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using television/computer >2 h/day (1.63; 1.23–2.17) and being a current smoker (2.46; 1.80–3.35) or snuffer (2.11; 1.57–2.85). Furthermore, those who reported short time in bed had higher odds of having an irregular intake of breakfast (2.24; 1.77–2.84), lunch (1.57; 1.25–1.97), dinner (1.87; 1.46–2.40) and evening meals (1.44; 1.13–1.85) and an irregular intake of all four meals (1.79; 1.32–2.43). Finally, short time in bed was associated with lower odds of intake of sweets/candy P4 times/week (0.51; 0.32–0.83) and higher odds of poor academic achievement (1.62; 1.26–2.09). The association of short time in bed and overweight/obesity, multiple health risk behaviors and academic achievement, varied by sex for three of the 13 associations tested (data not shown). The association of short time in bed with an irregular evening meal was significant among boys (AOR, 1.57; 95% CI, 1.14–2.16), but not among girls (1.31; 0.89–1.92) and the association of short time in bed with an intake of sweets/candy P4 times/week was significant among girls (0.36; 0.17–0.79), but not among boys (0.66; 0.36– 1.24). Likewise, the association of short time in bed with low school grades was significant among girls (2.14; 1.46–3.12), but not among boys (1.22; 0.86–1.72).
4. Discussion Despite increasing evidence that short sleep duration is related to several health-risk behaviors, poor health and poor academic achievement, the results from previously published studies are partly inconclusive, and in contrast to the present study, few other studies targeting adolescents have a very high participation rate similar to the present study. Based on our results on self-reported sleep, short time in bed (<8 h/day) indicated short sleep duration/insufficient sleep. This study showed that almost one in three Norwegian adolescents (32.3%) reported short time in bed (<8 h/day) on an average school night. Another study among older Norwegian adolescents (16–19 years) revealed that 39.2% slept <7 h on weekdays [49], whereas the National Sleep Foundation has reported that 63–67% of American adolescents (15–17 years) slept <8 h/day [3]. One study reported significant differences in sleep duration in children from eight European countries (range from 9.5 to 11.2 h/day), and the authors concluded that regional affiliation, including culture and environmental characteristics, seems to influence sleep duration [50]. Most likely, the variation in results can also be explained by differences in definitions. Some studies have defined sleep duration by subtracting reported wake time whereas other studies have, similar to the present study, based their results on time in bed, and thereby most likely overestimated sleep duration [51]. Hysing et al. [52] revealed large discrepancies between reported mean time in bed (7:29 h/day) and mean sleep duration (6:25 h/day) for Norwegian adolescents. Furthermore, studies have reported that seasonal variation may have an impact on our sleep–wake cycle [53,54], which may partly explain the observed discrepancies between findings in the present study and previous studies. On the other hand, the seasonal effect on sleep duration may be limited, as a recently published Norwegian study showed no evidence of seasonal effect on insomnia or time in bed among adolescents [55]. Results from the present study also showed that girls reported significantly longer time in bed than boys, with a higher frequency of boys reporting short time in bed (<8 h) on an average school night compared with girls. This result is in line with another recently published Norwegian study among adolescents (aged 16–18 years) [52]. However, this latter study and studies among American adolescents have reported the opposite trend among sexes when considering sleep duration [3,23].
No relationship was detected between time in bed and overweight and/or obesity in the present study. On the other hand, most previously published cross-sectional studies have shown that short sleep duration was associated with high BMI and prevalence of obesity in both children and adolescents [20–22]. Results from the HELENA study, which targeted European adolescents, showed that sleeping <8 h was associated with an increased BMI, body fat, waist and hip circumferences and fat mass index [21]. In line with these latter results, another study among older Norwegian adolescents (aged 16–19 years) reported that sleeping <7 h was associated with an increased BMI [49]. Longitudinal studies, however, have reported inconsistent results, as some confirm an inverse association [25–27], whereas other studies have not detected any association between sleep duration and obesity [28,29]. Differences in definitions of short time in bed/short sleep duration or study methods may partially explain the inconsistent results. Furthermore, a stronger association between sleep and overweight/obesity has been shown in children and adults [56] than in adolescents [29], which may reflect a more complex relationship between sleep and body weight in adolescence due to the onset of puberty. In general, short time in bed duration was associated with an irregular meal pattern, including all four main meals (breakfast, lunch, dinner and evening meal) in the present study. Few other studies have focused on the association between sleep duration and meal skipping among adolescents, although a recently published study found a significant association between sleeping P8 h/day and a high frequency of breakfast intake in Arab adolescents [57]. The authors of the latter study argue that the findings are expected, as those with long sleep duration can afford to wake up early enough to have breakfast before school starts, whereas other studies have confirmed that short sleep duration is associated with skipping breakfast and lunch meals in pre-adolescents (9–11 years) [33] and adults [58–60]. Analyses from the present study showed that girls, but not boys, reporting short time in bed had significantly lower odds of consuming sweets and candy P4 times/week compared with those reporting a sleep duration P8 h/day. No significant associations were detected between time in bed and the intake of soft drinks and salty snacks in the present study. A study among German children and adolescents has also reported a higher intake of sweets and fast food in girls, as well as a higher intake of soft drinks in boys sleeping P8 h/day [61]. Additionally, Arab teens sleeping P8 h/day have reported a more frequent intake of french fries/ potato chips, cake/donuts, sugar-sweetened beverages and sweets [57]. Studies among American adolescents have also confirmed an association between short sleep duration and the consumption of soft drinks [35,36]. On the other hand, the HELENA study detected no association between short sleep duration and the consumption of sweets or soft drinks among a large population of European adolescents [21]. A recently published study reported that late bedtimes and late wake-up times were associated with poorer diet quality, independent of sleep duration, level of physical activity and child and sociodemographic factors [62]. The present study showed that a short time in bed was associated with spending >2 h/day watching television or using a computer. This is in accordance with previously published studies on adolescents, which found a short sleep duration to be associated with an increased amount of time spent watching television [21,63] and using a computer [36,37,63]. A recently published study reported that the more interactive technological devices used in the hour before bed, the more likely difficulties falling asleep and poor sleep were reported [64]. This result is not surprising as studies have reported that high screen-time increases the activity in the nervous system, causing alertness and physiological arousal, making it more difficult to fall asleep [65,66].
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According to both international and national recommendations, children and adolescents should engage in moderate to vigorous physical activity for P60 min/day [67]. Adolescents reporting short time in bed in the present study had significantly higher odds of failing to meet these recommendations compared with those reporting P8 h sleep on an average school day. These latter results are in line with studies among both American and European adolescents using the same cut-off for the categorization of short sleep duration (<8 h) [21,36]. It is likely that short sleep duration increases the feeling of tiredness and lack of energy, which may partially explain low levels of physical activity. Even so, a study by Foti et al. [37] exhibited a significant association between obtaining P8 h sleep and daily physical activity for P60 min, though not with vigorous physical activity. The description of physical activity reported in the present study is similar to the description of vigorous physical activity used in the latter study [37]; hence if additional questions concerning lower intensity levels of physical activity had been included in the present study, sleep duration and activity habits might have demonstrated a stronger association. This hypothesis is supported by Brand et al. [68], who confirmed that regular but not necessarily vigorous exercise was related to improvement in objective sleep patterns among adolescents. Furthermore, vigorous-intensity activity close to bedtime has been found to interfere negatively with sleep [69]. Results from this study also showed that short time in bed was associated with higher odds of smoking, which is in accordance with previously published studies among adolescents [36,38]. The present study also revealed a strong association between short time in bed and the use of snuff, and to the best of our knowledge no other studies have investigated the relationship between sleep duration and the use of snuff among adolescents. However, a study among American adults showed that those who were currently using snuff or chewing tobacco had twice the odds of short sleep duration compared with the rest of the study sample [43]. The relationship between sleep pattern, smoking and snuffing may be explained in part by the intake of nicotine, which has been shown to cause sleep disturbances, including longer sleep onset, lighter sleep, decreased sleep efficiency and increased daytime sleepiness, through the stimulation of sleep-regulating neurotransmitters [70–73]. A strong association was shown between short time in bed and low school grades in the present study, but further analyses revealed that this trend was only significant for girls, not for boys. Few similar studies have reported sex differences, although a meta-analysis confirmed that sleep measurements had different degrees of contributions to academic achievement, with sleepiness being the strongest determinant, followed by sleep quality and sleep duration [46]. This could indicate that the individual experience of insufficient sleep, such as sleepiness and lack of concentration, is affecting academic achievement to a higher degree than sleep duration. A recently published study reported that a short sleep duration resulted in reduced attention due to daytime somnolence, which in turn may negatively affect academic achievement [61], whereas another study among Finnish adolescents confirmed that the later the bedtime, the lower the motivation and academic achievement [74]. A major strength of the present study was the high participation rate (98%), whereas a few other studies have investigated the association between sleep duration and a variety of health-related lifestyle factors and academic achievement in this target group. However, the cross-sectional design of this study does not allow the identification of causal effects. In addition, the results from this study were based on self-reported data; nonetheless, the questions in the questionnaire have demonstrated good test–retest reliability. Moreover, the present study did not include measurements
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of sleep quality and the experience of sleepiness, and was also unable to control for individuals’ sleep requirements and that those reporting short time in bed actually experienced the perceived consequences of insufficient sleep. In addition, the study lacks data on variations in time in bed through the week, including weekends. 5. Conclusion A high number of adolescents (32.3%) reported short time in bed on an average school night. Furthermore, the results from the study showed that short time in bed was associated with several health-risk behaviors, including irregular meal patterns, high intake of candy/sweets, high screen time, low level of physical activity, and smoking and snuffing. In addition, short time in bed was associated with poor academic achievement. Based on these results, future health-promoting programs should include a focus on increased sleep duration in adolescents. Funding sources Aust-Agder and Vest-Agder County, and the Norwegian Olympic and Paralympic Committee and Confederation of Sports (NIF). Conflict of interest The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: http://dx.doi.org/10.1016/j.sleep.2014.01.019.
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Sleep Medicine journal homepage: www.elsevier.com/locate/sleep
Original Article
Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents T.H. Stea ⇑, T. Knutsen, M.K. Torstveit University of Agder, Faculty of Sport and Health Sciences, Kristiansand, Norway
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 10 January 2014 Accepted 11 January 2014 Available online xxxx Keywords: Adolescents Sleep duration Overweight Obesity Health-risk behavior Academic achievement
a b s t r a c t Objective: To investigate the prevalence of short time in bed (<8 h/day) and to examine the association between time in bed, overweight/obesity, health-risk behaviors and academic achievement in adolescents. Methods: This study included a sample of adolescents (n = 2432) aged 15–17 years in the southern part of Norway (participation rate, 98.7%). A self-report questionnaire was used to assess time in bed, body mass index, dietary habits, physical activity habits, sedentary behavior, smoking and snuffing habits, and academic achievement. Results: A total of 32.3% of the students reported short time in bed (<8 h/day) on an average school night. Several health-risk behaviors were associated with short sleep duration, including not being physically active for P60 min for P5 days/week (adjusted odds ratio, 1.33; 95% confidence interval, 1.05–1.68); using television/computer >2 h/day (1.63; 1.23–2.17); being a current smoker (2.46; 1.80–3.35) or snuffer (2.11; 1.57–2.85); having an irregular meal pattern (1.33; 1.05–1.68); intake of sweets/candy P4 times/week (0.51; 0.32–0.83); and poor academic achievement (1.62; 1.26–2.09). All odds ratios were adjusted for sex, age and parental education. Conclusions: In Norwegian adolescents, short time in bed is associated with several health-risk behaviors and poor academic achievement. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Adolescents’ sleep duration has decreased over the past century, and research has also indicated that the pattern of decreased sleep duration increases with age [1,2]. Studies have estimated that most adolescents need 8–9 h of sleep per night [3,4], and the National Sleep Foundation, a US-based non-profit organization, has estimated that most adolescents need 9 h of sleep, and that <8 h of sleep on weeknights is characterized as insufficient sleep duration [3]. Although European guidelines for sleep duration are lacking, several studies have shown that short sleep duration (<8 h/day) is associated with several negative health consequences such as high blood pressure [5,6], insulin resistance and diabetes [7–9], decreased immune response [10,11] and mental health problems [12–18]. If left untreated, sleep disturbances at age 16 years have been shown to predict sleep disturbances in adulthood [19].
⇑ Corresponding author. Address: University of Agder, Faculty of Sport and Health Sciences, PO Box 422, 4604 Kristiansand, Norway. Tel./fax: +47 38142324. E-mail address: [email protected] (T.H. Stea).
Several cross-sectional studies have shown that short sleep duration is associated with high body mass index (BMI; kg/m2) and a prevalence of obesity in children and adolescents [20–22]. Some studies suggest, however, that these associations are apparent only in girls, not in boys [23,24]. Longitudinal studies have also reported conflicting results; some confirm an inverse association [25–27], whereas other studies have not detected any association between sleep duration and obesity [28,29]. Moreover, another longitudinal study reported that the association between sleep duration and BMI varied by sex and age, with stronger associations in boys than girls and in younger children compared with adolescents [30]. Previous studies have also shown inconsistent findings regarding the association between sleep duration and food intake, with a study reporting that one night of reduced sleep subsequently increased food intake among young healthy men [31]. However, a recent study reported that sleep restriction was associated with a small negative energy balance driven by an increased energy expenditure from prolonged wakefulness, and a concomitant decreased energy intake and motivation to eat in male teenagers [32]. Other studies have shown an association between short sleep duration and low meal frequency [33], high intakes of snacks [34]
http://dx.doi.org/10.1016/j.sleep.2014.01.019 1389-9457/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Stea TH et al. Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents. Sleep Med (2014), http://dx.doi.org/10.1016/j.sleep.2014.01.019
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and soft drinks [35,36], as well as low intakes of fruits and vegetables among children and adolescents [21]. Several studies have reported an association between sleep duration and screen time, whereas short sleep duration has been associated with using a computer P2–3 h per/day [36,37] and watching television >2 h per/day in adolescence [21]. In addition, several studies have shown that short sleep duration is associated with low levels of physical activity in high school students [21,36,37]. Furthermore, smoking cigarettes in adolescence has been associated with insufficient sleep on weekdays [36,38], inconsistent sleep patterns between weekdays and weekends [39,40], delayed sleep phase and other sleep disorders [41,42]. Few studies have focused on the relationship between the use of snuff and sleeping habits in adolescent years; however, a study targeting an adult population reported that the use of snuff has increased the odds of insufficient sleep compared with those not using these tobacco products [43]. Insufficient sleep duration may reduce the ability to concentrate and retain information [44], and shortened sleep duration and delayed sleep have been inversely associated with school performance [33,41,45]. A meta-analysis by Dewald et al. [46] also concluded that sleepiness and a low sleep quality were associated with a poor school performance. Nevertheless, other studies have reported no association between sleep duration and academic achievement in adolescents [38,47]. Because previous studies have shown inconsistent results, there is a need for additional research on sleep habits and associations with negative health consequences, health-risk behaviors and school performance. Thus, the main aims of the present study were to investigate the prevalence of short sleep duration (<8 h), and to examine possible associations between short sleep duration, overweight/obesity, several health-risk behaviors and academic achievement in Norwegian adolescents. We hypothesized that short time in bed is associated with overweight/obesity and the following health-risks behaviors: low physical activity level, high screen time, smoking and snuffing, an irregular meal pattern and high intake of unhealthy food and beverages, as well as poor academic achievement.
2. Methods 2.1. Study design and participants This cross-sectional study is part of a large, school-based cluster-randomized intervention study, ‘Active and Healthy Youth’, that promotes a healthy diet and activity patterns, and aims to prevent disordered eating among adolescents. The target group was 15–17-year-old high school students living in the southern part of Norway, and in agreement with school boards/school principals, a total of 17 out of 23 schools (73.9%) decided to participate in the present study. The main reason why the six schools did not want to participate was due to lack of time (five schools) and participation in similar public health projects (one school). A total of 2619 out of 2653 eligible students agreed to participate (98.7%) and the data collection was conducted in the period from October 2010 to February 2011. During the data analyses, a total of 187 students were excluded because they did not meet the age requirements for participation. Hence the data analyses in the present study were based on a total number of 2432 participants between the ages of 15 and 17 years. The Regional Committee for Medical Research Ethics approved the study protocol, and written consents were obtained from all students prior to participation in this study.
2.2. Measurement methods and procedures The participants were asked to complete a 15-page questionnaire that included questions about bedtime and wake time, body weight, height, exercise habits, screen time activity, smoking and snuff use, dietary intake, meal frequencies and paternal and maternal education level. In order to test the reliability of the questions used in the present study, the questionnaire was test–retested in a method study among 143 adolescents, aged 15–17 years. Test–retest reliability was measured by the intraclass coefficient (ICC). The questions included in this study demonstrated good test–retest reliability with a correlation coefficient ranging from 0.68 to 0.99. The students used 30 min answering the self-reporting questionnaire, with at least one member of the project team present to provide information about the project and answer possible questions. Sleep duration was assessed by asking: ‘When do you usually get out of bed on a school day?’ and ‘When do you usually go to bed on a school night?’ on a typical weekday. These items were used to compute the average total hours of daily time in bed, and responses were dichotomized into <8 h (short time in bed) and P8 h. The ICC (95% CI) was 0.83 (0.75–0.89) for bed time and 0.73 (0.61–0.82) for wake time. Self-reported weight and height were used to calculate BMI; categories were calculated using sex- and age-specific International Obesity Task Force (IOTF) cut-off points for defining overweight and obesity in children and adolescents aged 2–18 years [48], though, due to small numbers, the overweight and obese groups were collapsed into one group. The ICC (95% CI) was 0.98 (0.98–0.99) for weight and 0.99 (0.98–0.99) for height. Diet was assessed by a food frequency questionnaire of 13 items, including both food items and drinks. The questions had eight different response alternatives, ranging from never to more than once a day, and the response alternatives were further dichotomized into having a high or low intake of the selected food items and drinks. Having soft drinks, sweets and candy and a salty snack P4 times a week, respectively, was categorized as a high intake. The ICC (95% CI) was 0.85 (0.77–0.90) for soft drinks, 0.80 (0.70– 0.86) for sweets and candy and 0.75 (0.63–0.83) for salty snacks. Meal frequency was assessed by questions such as: ‘How often do you have breakfast each week?’, with the same asked for lunch, dinner and the evening meal. Response alternatives ranged from never or seldom to seven days a week, and in a statistical analysis the numbers of days reported eating breakfast, lunch, dinner and evening meals were then dichotomized into having an irregular meal pattern (omitted these main meals at least once a week) and having a regular meal pattern (intake of these main meals every day). These dichotomous variables were then combined to create a summary variable referred to as ‘irregular all four meals’, i.e. those skipping one of the main meals at least once a week versus those eating all meals every day. The ICC (95% CI) was 0.91 (0.86–0.94) for breakfast, 0.78 (0.68–0.85) for lunch, 0.68 (0.55– 0.79) for dinner and 0.89 (0.83–0.93) for evening meal. Physical activity level was estimated by asking: ‘How many hours per week do you spend on doing sports/physical activity in a way that makes you breathless or sweat?’ Response alternatives were: ‘0 h, 1–2 h, 3–4 h, 5–7 h, 8–10 h and 11 h or more.’ For the statistical analysis, not participating in 60 min of physical activity every day was characterized as health-risk behavior. Thus, the response alternatives were dichotomized into participating in 60 min of physical activity every day or more versus not engaging in recommended levels of physical activity (<60 min of physical activity every day). The ICC (95% CI) was 0.91 (0.87–0.93) for physical activity. Television viewing and computer use were enquired about as follows: ‘Excluding school hours on a regular weekday, how many
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hours do you watch TV or using PC/games?’ Response alternatives were: ‘I don’t watch TV or do gaming activity on a regular weekday, less than an hour a day, one hour a day, two hours a day, three hours a day, four hours a day or five hours or more a day.’ This variable was dichotomized into <2 h/day and P2 h/day, according to the American Academy of Pediatrics’ recommendation of screen time limits for children and adolescents [16]. The ICC was 0.81 (95% CI, 0.72–0.88) for television viewing and computer use. Smoking and snuffing habits were assessed by the question: ‘Do you smoke/use snuff?’ The response alternatives were: ‘Have never smoked/snuffed; have tried smoke/snuff, but not anymore, have smoked/snuffed regularly, but not anymore; smoking/snuffing, but not regularly and smoking/snuffing regularly and about . . . cigarettes/day.’ For the statistical analysis, those who reported smoking or snuffing occasionally or daily were classified as being a smoker/snuffer. The ICC (95% CI) was 0.95 (0.92–0.97) for smoking and 0.93 (0.90–0.96) for snuffing. The parental educational level was assessed with the question: ‘What level of education do your parents have?’ The question had four response alternatives: (i) elementary school, (ii) high school, (iii) college or university (63 years), and (iv) college or university (>3 years). These response alternatives were then dichotomized into lower and higher education levels (lower: no college or university education; higher: having attended college or university). The ICC (95% CI) was 0.83 (0.77–0.88) for maternal education and 0.80 (0.72–0.85) for paternal education. Achieved academic achievement was enquired about as follows: ‘In the last semester, what were your achieved grades in mathematics, English and Norwegian?’ A six-point grading scale (range, 0–6) was used to evaluate academic achievement, in which 6 was the best grade possible to obtain in Norwegian high schools. For statistical analyses, the mean grade was calculated and the results were dichotomized into poor (range, 0–3) and adequate (range, 4–6) academic achievement scores. The ICC (95% CI) was 0.82 (0.73–0.88) for achieved grades in mathematics, 0.83 (0.75– 0.89) for achieved grades in English, and 0.76 (0.65–0.84) for achieved grades in Norwegian.
Fig. 1. Distribution of sleep duration on school nights, by gender. The number of adolescents reporting short time in bed (<8 h) is presented by the bars representing 66 h and 7 h in bed, respectively.
calculate differences in overweight, physical activity, television/ computer use, smoking and snuffing habits, meal pattern and dietary intake between those reporting short time in bed (<8 h/ day) and adequate time in bed (P8 h/day). Multiple logistic regressions were used to explore the relationship between short sleep duration and the health-risk behaviors mentioned above, and the regression model was adjusted for age, sex, and parental education. Odds ratios were presented with 95% confidence intervals (CIs), and two-tailed P < 0.05 was considered statistically significant. 3. Results The mean time in bed on an average school night was 7.8 h (95% CI, 7.8–7.9) and appears as a bell-shaped curve for both sexes (Fig. 1). Approximately 32% of the students reported short time in bed (<8 h/day) on an average school night, whereas the number was significantly higher among boys (36%) than among girls (29%) (P < 0.001). Table 1 presents overweight/obesity, multiple health-risk behaviors and academic achievement relative to time in bed (<8 h/day vs. P8 h/day) adjusted for age, sex and parental education. The results did not show any association between time in bed and overweight/obesity, nor when analyzing the overweight group and the obese group separately. Those reporting short time in bed had higher odds of not participating in 60 min of daily physical activity (adjusted odds ratio [AOR], 1.33; 95% CI, 1.05–1.68),
2.3. Statistical analysis All statistical analyses were performed using SPSS version 19.0 (SPSS Inc., Chicago, IL, USA). In Table 1, the results are presented as a percentage of participants and v2-tests were performed to
Table 1 Prevalence and adjusted odds ratio (AOR) of overweight/obesity, selected health-risk behaviors and school grades by sleep duration.
Overweight/obeseb Did not participate in 60 min of physical activity every day TV/PC (>2 h/day) Current smokerc Current snufferc Irregular breakfast Irregular lunch Irregular dinner Irregular evening meal Irregular meal patternc (<4 meals per day) Regular soft drinks (P4 times a week) Sweet and candy (P4 times a week) Salty snack (P4 times a week) Low school gradesd
AORa (P8 h is the referent)
95% CI
p-value
16.3–20.4 55.9–60.9
1.22 1.33
0.91–1.65 1.05–1.68
0.191 0.018
73.1–77.3 10.7–13.9 10.9–14.2 31.0–35.7 44.8–49.8 23.1–27.5 59.9–64.4 75.1–79.4 26.7–34.5 10.6–16.4 10.6–16.4 26.9–42.9
1.63 2.46 2.11 2.24 1.57 1.87 1.44 1.79 0.79 0.51 0.82 0.80
1.23–2.17 1.80–3.35 1.57–2.85 1.77–2.84 1.25–1.97 1.46–2.40 1.13–1.85 1.32–2.43 0.52–1.18 0.32–0.83 0.46–1.46 0.71–0.91
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.004 <0.001 0.249 0.007 0.494 0.001
Time in bed <8 h
Time in bed P8 h
%
95% CI
%
95% CI
20.3 64.7
17.1–23.4 61.2–68.2
18.4 58.4
83.8 25.7 24.6 50.7 60.1 37.7 67.8 84.2 28.9 18.6 12.4 42.9
81.1–86.4 22.5–28.8 21.5–27.8 47.1–54.3 56.5–63.6 34.2–41.2 64.4–71.2 81.5–86.9 23.5–34.4 13.9–23.2 13.9–23.2 39.3–46.5
75.2 12.3 12.6 33.4 47.3 25.3 62.3 77.2 30.6 13.5 13.5 34.9
a
Adjusted for age, sex and parental education. Based on self-reported height and weight, body mass index (BMI = weight [kg]/height [m]2) P25, using sex and age specific International Obesity Task Force (IOTF) cut-off points. c Self-reported smoking/snuffing sometimes or daily. d Mean grade in mathematics, English and Norwegian language between 0 and 3 on a 6 point grading scale (0–6). b
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using television/computer >2 h/day (1.63; 1.23–2.17) and being a current smoker (2.46; 1.80–3.35) or snuffer (2.11; 1.57–2.85). Furthermore, those who reported short time in bed had higher odds of having an irregular intake of breakfast (2.24; 1.77–2.84), lunch (1.57; 1.25–1.97), dinner (1.87; 1.46–2.40) and evening meals (1.44; 1.13–1.85) and an irregular intake of all four meals (1.79; 1.32–2.43). Finally, short time in bed was associated with lower odds of intake of sweets/candy P4 times/week (0.51; 0.32–0.83) and higher odds of poor academic achievement (1.62; 1.26–2.09). The association of short time in bed and overweight/obesity, multiple health risk behaviors and academic achievement, varied by sex for three of the 13 associations tested (data not shown). The association of short time in bed with an irregular evening meal was significant among boys (AOR, 1.57; 95% CI, 1.14–2.16), but not among girls (1.31; 0.89–1.92) and the association of short time in bed with an intake of sweets/candy P4 times/week was significant among girls (0.36; 0.17–0.79), but not among boys (0.66; 0.36– 1.24). Likewise, the association of short time in bed with low school grades was significant among girls (2.14; 1.46–3.12), but not among boys (1.22; 0.86–1.72).
4. Discussion Despite increasing evidence that short sleep duration is related to several health-risk behaviors, poor health and poor academic achievement, the results from previously published studies are partly inconclusive, and in contrast to the present study, few other studies targeting adolescents have a very high participation rate similar to the present study. Based on our results on self-reported sleep, short time in bed (<8 h/day) indicated short sleep duration/insufficient sleep. This study showed that almost one in three Norwegian adolescents (32.3%) reported short time in bed (<8 h/day) on an average school night. Another study among older Norwegian adolescents (16–19 years) revealed that 39.2% slept <7 h on weekdays [49], whereas the National Sleep Foundation has reported that 63–67% of American adolescents (15–17 years) slept <8 h/day [3]. One study reported significant differences in sleep duration in children from eight European countries (range from 9.5 to 11.2 h/day), and the authors concluded that regional affiliation, including culture and environmental characteristics, seems to influence sleep duration [50]. Most likely, the variation in results can also be explained by differences in definitions. Some studies have defined sleep duration by subtracting reported wake time whereas other studies have, similar to the present study, based their results on time in bed, and thereby most likely overestimated sleep duration [51]. Hysing et al. [52] revealed large discrepancies between reported mean time in bed (7:29 h/day) and mean sleep duration (6:25 h/day) for Norwegian adolescents. Furthermore, studies have reported that seasonal variation may have an impact on our sleep–wake cycle [53,54], which may partly explain the observed discrepancies between findings in the present study and previous studies. On the other hand, the seasonal effect on sleep duration may be limited, as a recently published Norwegian study showed no evidence of seasonal effect on insomnia or time in bed among adolescents [55]. Results from the present study also showed that girls reported significantly longer time in bed than boys, with a higher frequency of boys reporting short time in bed (<8 h) on an average school night compared with girls. This result is in line with another recently published Norwegian study among adolescents (aged 16–18 years) [52]. However, this latter study and studies among American adolescents have reported the opposite trend among sexes when considering sleep duration [3,23].
No relationship was detected between time in bed and overweight and/or obesity in the present study. On the other hand, most previously published cross-sectional studies have shown that short sleep duration was associated with high BMI and prevalence of obesity in both children and adolescents [20–22]. Results from the HELENA study, which targeted European adolescents, showed that sleeping <8 h was associated with an increased BMI, body fat, waist and hip circumferences and fat mass index [21]. In line with these latter results, another study among older Norwegian adolescents (aged 16–19 years) reported that sleeping <7 h was associated with an increased BMI [49]. Longitudinal studies, however, have reported inconsistent results, as some confirm an inverse association [25–27], whereas other studies have not detected any association between sleep duration and obesity [28,29]. Differences in definitions of short time in bed/short sleep duration or study methods may partially explain the inconsistent results. Furthermore, a stronger association between sleep and overweight/obesity has been shown in children and adults [56] than in adolescents [29], which may reflect a more complex relationship between sleep and body weight in adolescence due to the onset of puberty. In general, short time in bed duration was associated with an irregular meal pattern, including all four main meals (breakfast, lunch, dinner and evening meal) in the present study. Few other studies have focused on the association between sleep duration and meal skipping among adolescents, although a recently published study found a significant association between sleeping P8 h/day and a high frequency of breakfast intake in Arab adolescents [57]. The authors of the latter study argue that the findings are expected, as those with long sleep duration can afford to wake up early enough to have breakfast before school starts, whereas other studies have confirmed that short sleep duration is associated with skipping breakfast and lunch meals in pre-adolescents (9–11 years) [33] and adults [58–60]. Analyses from the present study showed that girls, but not boys, reporting short time in bed had significantly lower odds of consuming sweets and candy P4 times/week compared with those reporting a sleep duration P8 h/day. No significant associations were detected between time in bed and the intake of soft drinks and salty snacks in the present study. A study among German children and adolescents has also reported a higher intake of sweets and fast food in girls, as well as a higher intake of soft drinks in boys sleeping P8 h/day [61]. Additionally, Arab teens sleeping P8 h/day have reported a more frequent intake of french fries/ potato chips, cake/donuts, sugar-sweetened beverages and sweets [57]. Studies among American adolescents have also confirmed an association between short sleep duration and the consumption of soft drinks [35,36]. On the other hand, the HELENA study detected no association between short sleep duration and the consumption of sweets or soft drinks among a large population of European adolescents [21]. A recently published study reported that late bedtimes and late wake-up times were associated with poorer diet quality, independent of sleep duration, level of physical activity and child and sociodemographic factors [62]. The present study showed that a short time in bed was associated with spending >2 h/day watching television or using a computer. This is in accordance with previously published studies on adolescents, which found a short sleep duration to be associated with an increased amount of time spent watching television [21,63] and using a computer [36,37,63]. A recently published study reported that the more interactive technological devices used in the hour before bed, the more likely difficulties falling asleep and poor sleep were reported [64]. This result is not surprising as studies have reported that high screen-time increases the activity in the nervous system, causing alertness and physiological arousal, making it more difficult to fall asleep [65,66].
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T.H. Stea et al. / Sleep Medicine xxx (2014) xxx–xxx
According to both international and national recommendations, children and adolescents should engage in moderate to vigorous physical activity for P60 min/day [67]. Adolescents reporting short time in bed in the present study had significantly higher odds of failing to meet these recommendations compared with those reporting P8 h sleep on an average school day. These latter results are in line with studies among both American and European adolescents using the same cut-off for the categorization of short sleep duration (<8 h) [21,36]. It is likely that short sleep duration increases the feeling of tiredness and lack of energy, which may partially explain low levels of physical activity. Even so, a study by Foti et al. [37] exhibited a significant association between obtaining P8 h sleep and daily physical activity for P60 min, though not with vigorous physical activity. The description of physical activity reported in the present study is similar to the description of vigorous physical activity used in the latter study [37]; hence if additional questions concerning lower intensity levels of physical activity had been included in the present study, sleep duration and activity habits might have demonstrated a stronger association. This hypothesis is supported by Brand et al. [68], who confirmed that regular but not necessarily vigorous exercise was related to improvement in objective sleep patterns among adolescents. Furthermore, vigorous-intensity activity close to bedtime has been found to interfere negatively with sleep [69]. Results from this study also showed that short time in bed was associated with higher odds of smoking, which is in accordance with previously published studies among adolescents [36,38]. The present study also revealed a strong association between short time in bed and the use of snuff, and to the best of our knowledge no other studies have investigated the relationship between sleep duration and the use of snuff among adolescents. However, a study among American adults showed that those who were currently using snuff or chewing tobacco had twice the odds of short sleep duration compared with the rest of the study sample [43]. The relationship between sleep pattern, smoking and snuffing may be explained in part by the intake of nicotine, which has been shown to cause sleep disturbances, including longer sleep onset, lighter sleep, decreased sleep efficiency and increased daytime sleepiness, through the stimulation of sleep-regulating neurotransmitters [70–73]. A strong association was shown between short time in bed and low school grades in the present study, but further analyses revealed that this trend was only significant for girls, not for boys. Few similar studies have reported sex differences, although a meta-analysis confirmed that sleep measurements had different degrees of contributions to academic achievement, with sleepiness being the strongest determinant, followed by sleep quality and sleep duration [46]. This could indicate that the individual experience of insufficient sleep, such as sleepiness and lack of concentration, is affecting academic achievement to a higher degree than sleep duration. A recently published study reported that a short sleep duration resulted in reduced attention due to daytime somnolence, which in turn may negatively affect academic achievement [61], whereas another study among Finnish adolescents confirmed that the later the bedtime, the lower the motivation and academic achievement [74]. A major strength of the present study was the high participation rate (98%), whereas a few other studies have investigated the association between sleep duration and a variety of health-related lifestyle factors and academic achievement in this target group. However, the cross-sectional design of this study does not allow the identification of causal effects. In addition, the results from this study were based on self-reported data; nonetheless, the questions in the questionnaire have demonstrated good test–retest reliability. Moreover, the present study did not include measurements
5
of sleep quality and the experience of sleepiness, and was also unable to control for individuals’ sleep requirements and that those reporting short time in bed actually experienced the perceived consequences of insufficient sleep. In addition, the study lacks data on variations in time in bed through the week, including weekends. 5. Conclusion A high number of adolescents (32.3%) reported short time in bed on an average school night. Furthermore, the results from the study showed that short time in bed was associated with several health-risk behaviors, including irregular meal patterns, high intake of candy/sweets, high screen time, low level of physical activity, and smoking and snuffing. In addition, short time in bed was associated with poor academic achievement. Based on these results, future health-promoting programs should include a focus on increased sleep duration in adolescents. Funding sources Aust-Agder and Vest-Agder County, and the Norwegian Olympic and Paralympic Committee and Confederation of Sports (NIF). Conflict of interest The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: http://dx.doi.org/10.1016/j.sleep.2014.01.019.
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Please cite this article in press as: Stea TH et al. Association between short time in bed, health-risk behaviors and poor academic achievement among Norwegian adolescents. Sleep Med (2014), http://dx.doi.org/10.1016/j.sleep.2014.01.019