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Sleep–wake habits and circadian preference in Mexican secondary school
Accepted Manuscript Title: Sleep–wake habits and circadian preference in Mexican secondary school Author: Arturo Arrona-Palacios, Aída García, Pablo Valdez PII: DOI: Reference:
S1389-9457(15)00856-4 http://dx.doi.org/doi: 10.1016/j.sleep.2015.05.026 SLEEP 2829
To appear in:
Sleep Medicine
Received date: Revised date: Accepted date:
25-3-2015 15-5-2015 29-5-2015
Please cite this article as: Arturo Arrona-Palacios, Aída García, Pablo Valdez, Sleep–wake habits and circadian preference in Mexican secondary school, Sleep Medicine (2015), http://dx.doi.org/doi: 10.1016/j.sleep.2015.05.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Original Article Sleep–wake habits and circadian preference in Mexican secondary school Arturo Arrona-Palaciosa*, Aída Garcíaa, Pablo Valdeza *Corresponding author at: Laboratory of Psychopshysiology, School of Psychology, Universidad Autónoma de Nuevo León, Mutualismo 110, Col Mitras Centro, Monterrey, Nuevo León 64460, México. Tel.: +52 81 83483866. E-mail address: [email protected] (A. Arrona-Palacios).
Highlights
The effects of sleep-wake habits and chronotype on school shifts were explored. Students from the morning shift have short sleep duration. Students from the morning shift with an evening type have shorter sleep duration. Students from the afternoon shift have adequate sleep duration.
ABSTRACT Objective: The current study aimed to determine the differences between sleep–wake habits and circadian preference in Mexican adolescents attending classes at a morning shift or an afternoon shift. Methods: A sample of 568 students of a secondary school in Reynosa, northeastern Mexico, of whom 280 were boys and 288 were girls (mean age 14.08 ± 0.72 years, age range 13–16 years). 1
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In the morning shift, 287 students attend classes on a schedule from 7:30 to 13:00 and the afternoon shift, 281 students, on a schedule from 13:20 to 19:00 Students completed a general information questionnaire, the Sleep Timing Questionnaire and the Spanish version of the Morningness–Eveningness Questionnaire. Results: The adolescents who attended the morning shift had earlier bedtime and waking time, but shorter sleep duration than those who attended the afternoon shift. Those oriented to eveningness had later bedtime, waking time, and a shorter sleep duration than those oriented to morningness. Two interactions were found between school shift and chronotype: first, with regard to waking time during weekdays, students who attended the afternoon shift and were oriented to eveningness woke up later than those who attended the morning shift and were oriented to eveningness; and second, during weekdays, there were no differences between the waking time of morning-type and evening-type students who attended the morning shift. With regard to sleep duration on weekdays, students who attended the morning shift and were oriented to eveningness have the shortest sleep duration. Furthermore, there were no differences between sleep duration on weekdays in evening-type and morning-type students of the afternoon shift. Conclusion: Adolescents who attend classes in the morning shift and are oriented to eveningness are the most sleep deprived. Those who attend the afternoon shift will have optimal sleep duration, regardless of their circadian preference. Keywords: Sleep habits; Adolescents; Chronotype; Sleep education; Mexico; School shifts 1. Introduction During adolescence, the school schedule is the most damaging environmental constraint in sleep habits, especially if is taken into account that the adolescent usually attends school at 07:30 in 2
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the morning throughout the week. On the other hand, a phase delay occurs in the sleep–wake Comment [KVS1]: AUTHOR: Please note that all references have been numbered in order of citation, in keeping with journal style.
cycle during adolescence [1], which makes it difficult to sleep early; and therefore adolescents often go to sleep after midnight. Thus, many adolescents get only 6 or less hours of sleep, thereby causing sleep deprivation [2]. Sleep duration is reduced on weekdays, as adolescents have to get up early to attend classes and tend to stay up late. However, on weekends, holidays, and vacations, adolescents often have longer sleep duration and go to sleep and wake up later [3– 5]. Thus, on weekdays, a sleep debt occurs in adolescents, which causes the appearance of certain sleep disorders such as daytime sleepiness and the delayed sleep phase syndrome, among others [6,7]. Sleep deprivation in adolescents has been associated with a deterioration in general health and immune function [8], increased risk of injuries and accidents [9], increased suicidal ideation [10,11], and increased use of drugs and alcohol [12,13], as well as deficiencies in motor skills [14], working memory [15,16], attention [6,17,18], and retention of information [19,20], with a decline in academic performance [21].. Currently there is a global concern about the increasing prevalence of sleep problems during adolescence [22]. Iglowstein et al. [23] found that the average duration of sleep in adolescents 11 to 16 years old decreased from 9.6 hours to 8.1 hours of sleep. In contrast, other studies suggest that teens are having shorter sleep duration, with an average of 7.6 hours per night [10,24]. In general, the diverse conditions from the social environment have a great influence on the reduction of sleep in adolescents [25,26]. Apart from school schedule, previous studies have indicated that the increasing amount of homework [7,27], lack of parental control [28,29], extracurricular activities [26,30,31], and recently the use of electronic media [32,33] contribute significantly to the adolescent sleeping less. However, one cannot ignore the fact that the need 3
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for sleep is not manifest at the same time in every person. The preference to perform a greater number of activities in the morning or at night is attributed to interindividual differences in the timing of circadian rhythms, which comprise as a continuum between the two extremes [34]. Individuals with a morning-type consitution have no problem with early rising, soon achieve their maximum of mental and physical activity, and become tired early in the evening. People who have an evening type tend to wake up late, become progressively more activate throughout the day, and are most active in the evening or at night; as a consequence, their need for sleep tends to manifest later compared with the that in morning-type individuals. Furthermore, there is also an intermediate type; these individuals fit into an intermediate portion between the two ends of the continuum, and constitute the largest part of the population [35,36]. Adolescents change from morning type to evening type from 12 to 14 years of age accordingly to their own biological changes at this age, as changes in their social environment occur [37]. With regard to gender, boys tend to be more oriented to eveningness than girls [38,39]. Nevertheless, the results of the gender differences in sleep patterns are contradictory. Several studies have found no gender differences at bedtime (weekdays/weekends) and waking time on weekdays [40,41]. Nevertheless, other studies have found that girls wake up earlier on weekdays but later on weekends [7,42], and go to bed earlier on weekdays [40,41,43] than boys. There are also cultural differences, but these are mediated by social, biological, and environmental factors [44,45]. Being an evening type has been identified in adolescents as a risk factor for behavioral and emotional problems such as consumption of alcohol, higher probability of smoking, caffeinated soft drinks, obesity, depression, minor psychiatric disorders, poor school performance and poor perceived quality of sleep [43,46,47,49]. The aforementioned risk factors are due to the divergence that occurs regarding the internal body clock and the calendar of social 4
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activities, especially for those who are evening-type persons. Therefore, these individuals have difficulty waking up early on workdays/weekdays (in this case, to attend classes), and they tend to take advantage of free days to rest and recover the loss of sleep on workdays [46,50]. Despite the cited reports, most data regarding sleep habits and circadian preferences of adolescents are based on research conducted in the United States and several countries in Europe and Oceania. There have been few studies that have examined sleep habits in a Mexican sample [51,52], and fewer with sleep habits of students attending school on different schedules [53]. The current study aimed to determine the differences between sleep–wake habits and circadian preference in Mexican adolescents attending classes during a morning shift or an afternoon shift.
2. Methods 2.1. Participants The final sample consisted of 568 students of a secondary school in Reynosa, northeastern Mexico (280 boys and 288 girls). The mean age of the overall sample was 14.08 ± 0.72 years, and the age range was between 13 and 16 years. In the morning shift, 287 students attended classes on a schedule from 7:30 to 13:00, and in the afternoon shift, 281 students attended classes on a schedule of 13:20 to 19:00. The Mexican secondary school consists of a 3-year program; students are distributed into 18 groups for the morning shift and 18 groups for the afternoon shift. Of the 18 groups, these are divided into six groups per school year for both shifts. For this study, a total sample size of 600 students was selected, which was partitioned into six groups, three for the morning shift and three for the afternoon shift (one group per school year). A total of 32 participants were excluded due to nonresponse or refusal to participate. The application 5
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had an estimated response time of 60 minutes; these questionnaires were applied at the same time during school classes for both shifts. Data collection was voluntary, and students received no payment of any kind. Each participant signed a letter of informed consent, as did the parents of participants who were underage. The research project was approved by the Academic Board of the University, and complied with the principles of the Declaration of Helsinki for research on human participants. 2.2. Measures 2.2.1. General information questionnaire A general information was used to obtain demographic information such as age, gender, school grade, and school shift. 2.2.2. Sleep Timing Questionnaire The Sleep Time Questionnaire is a single-administration instrument that allows determination of an individual’s habitual waking time and bedtime [54]. This questionnaire consists of 18 questions and is divided into two parts: the first section deals with sleeping habits during weekdays (school days), and the second section deals with sleeping habits during the weekend (free days). 2.2.3. Spanish version of the Morningness–Eveningness Questionnaire The Spanish version of the Morningness–Eveningness Questionnaire consists of 19 questions with a four-point Likert scale or a time of day scale. This questionnaire was designed to determine the temporary preference in the activities of individuals (chronotypes), classifying
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them according to their scores as morning types (59–86), intermediate types (42–58), and evening types (16–41) [35,55]. 2.3. Statistical analyses To evaluate differences for each dependent variable, a factorial analysis of variance (ANOVA) was performed, considering three factors, namely, school shift (morning and afternoon), gender, and circadian preference (morning type and evening type), with sleep variables (waking time, bedtime, and sleep duration) as dependent variables. The indicators that showed significant differences in the factorial ANOVA were subjected to a post hoc analysis using the Tukey honest significant difference (HSD) test.
3. Results 3.1. School shift No significant differences in gender were found in any sleep variable. Significant differences were found in the sleep–wake cycle between the morning shift and the afternoon shift, as well as on weekdays and weekends. During the week, participants in the morning shift went to bed earlier (morning shift: 23:00 ± 1:11; afternoon shift: 00:06 ± 1:36; F1,567 = 86.1, p < 0.001) and woke up earlier (morning shift: 06:11 ± 0:29; afternoon shift: 09:29 ± 1:20; F1, 567 = 1537.84, p < 0.001), but their total sleep duration was shorter (morning shift: 7:11 ± 1:13; afternoon shift: 9:23 ± 1:43; F1,567 = 309.83; p < 0.001) compared with participants in the afternoon shift. On weekends, the participants of the morning shift went to bed earlier (morning shift: 00:30 ± 1:51; afternoon shift: 01:12 ± 2:02; F1,567 = 17.60, p < 0.001) and woke up earlier (morning shift:
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10:30 ± 1:47; afternoon shift: 10:58 ± 1:57; F1,567 = 8.82, p < 0.01) compared with the afternoon shift. There was no significant difference in the total of sleep duration (morning shift: 10:00 ± 2:07; afternoon shift: 9:46 ± 2:17, F1,567 = 1.43, not significant) (Fig. 1). 3.2. Circadian preference The morning shift had a total of 47 evening types, 233 intermediate types, and 17 morning types, whereas the afternoon shift had a total of 59 evening types, 208 intermediate types, and 14 morning types. Significant differences between extreme chronotypes were found on weekdays and weekend sleeping habits. During weekdays, participants oriented to eveningness went to bed later (evening type: 24:22 ± 1:36; morning type: 22:32 ± 1:01 h; F2,567 = 25.95, p < 0.001), woke up later (evening type: 08:19 ± 2:12; morning type: 07:17 ± 1:37; F2,567 = 5.80, p < 0.01), and had a shorter sleep duration (evening type: 7:57 ± 2:15 h; morning type: 8:45 ± 1:28 h; F2,567 = 9.478, p < 0.001) than those oriented to morningness. During the weekend, participants who were oriented to eveningness went to bed later (evening type: 01:49 ± 1:56; morning type: 23:01 ± 1:20; F2,567 = 27.28, p < 0.001) and awoke later (evening type: 11:30 ± 1:50; morning type: 9:09 ± 1:42, F2,567 = 59.97, p < 0.001) than those oriented to morningness. No significant differences were found with regard to sleep duration (Fig. 2).
3.3 School shift and circadian preference interactions No significant interactions were found between the school shift and chronotype with bedtime on weekdays (Fig. 3A) and weekend (Fig. 3B). The analysis revealed a significant interaction between the school shift and chronotype with waking time on weekdays (F2,567 = 4.45; p < 0.01) (Fig. 3C). The effect of this interaction is that all the participants who attend classes in the 8
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morning shift must get up early to go to school, whereas those who attend classes in the afternoon shift have the opportunity to wake up at any time due to classes starting at 13:30. In the afternoon shift, the differences in waking time of those who are oriented to eveningness (09:59) and morningness (08:47) differ significantly (p < 0.001) during the weekdays. In contrast, participants from the morning shift who are oriented to eveningness have no significant differences in their waking time from participants oriented to morningness during these days. Furthermore, those who attend the afternoon shift and are oriented to eveningness wake up later (09:59) than those who attend the morning shift and are oriented to eveningness (06:12), with a difference of p < 0.001. No significant interactions were found between school shift and chronotype with regard to waking time on the weekend (Fig. 3D). In addition, a significant interaction between school shift and chronotype on sleep duration of weekdays was found (F2,567 = 4.499; p < 0.01) (Fig. 3E). This interaction is due to the fact that participants in the morning shift who are oriented to eveningness go to sleep later, but wake up early to attend classes, thus causing shorter sleep duration (6:16) compared to participants who are oriented to morningness (7:53), differing significantly (p < 0.001) during weekdays. In contrast, participants from the afternoon shift who are oriented to eveningness have no significant differences in their sleep duration from participants oriented to morningness. Furthermore, those who attend the morning shift and are oriented to eveningness have shorter sleep duration (6:16) than those who attend the afternoon shift and are oriented to eveningness (9:17) with a difference of p < 0.001. No significant differences were found between the school shift and chronotype with sleep duration on weekend (Fig. 3F).
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4. Discussion
This report provides information on sleep–wake habits and circadian preference among adolescents of two different school shifts—morning and afternoon shifts—in a Mexican population, which has rarely been examined both nationally and internationally [3]. Our main finding was that adolescents who attend classes in a morning shift tend to sleep less compared with those who attend classes in an afternoon shift. Our results also showed that adolescents who have a circadian preference for eveningness tend to sleep and wake up later, regardless of shift and gender. Those who have a circadian preference for eveningness and attend classes in the morning shift have the shortest sleep duration. In our study, adolescents who attended classes in the morning shift, slept during the week an average of 7 hours 11 minutes. When comparing the sleep habits of Mexican adolescents in this study to those of adolescents worldwide, Mexican adolescents slept about the same amount of hours as adolescents in Saudi Arabia [56,57], South Africa [58], Greece [59], Israel [60], and Taiwan [61]. On the other hand, Mexican adolescents slept fewer hours than reported in adolescents from different countries such as Spain [62], Australia [63], Germany [64], United States [65], China [66], and Brazil [67] where adolescents sleep approximately more than 8 hours. This suggests that adolescents attending classes in the morning shift have a duration of sleep below what is recommended (8.5–9.25 hours) [6,68,69]. The National Sleep Foundation [70] classifies sleep duration of adolescents into three levels as follows: recommended (8–10 hours), may be appropriate (7–11 hours), and not recommended (less than 7 and more than 11 hours); thus, the sleep duration of morning shift students is close to the inferior limit that may be
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appropriate, whereas students with an evening chronotype sleep and average of 6 hours and 16 minutes, making it a nonrecommended sleep duration for these students. These comparisons show the cultural differences in adolescents’ duration of sleep, as demonstrated in reviews by Gradisar et al. [3] and Olds et al. [71]. In addition to these cultural differences with adolescents who attend classes at a morning shift, there are differences also in the sleep habits of those who attend an afternoon shift, which has been only slightly explored because, in most countries, this type of school system (with a morning and afternoon shift) is not implemented. Adolescents who attended classes in an afternoon shift slept an average of 9 hours and 23 minutes during the week, which is slightly higher than has been reported in countries where the school system has an afternoon shift such as Croatia [72], Brazil [67,73], Greece [74], and even in an earlier study in Mexico [53] which report an average weekday sleep duration of 8 to 8.30 hours. Thus adolescents who have the opportunity to attend classes in an afternoon shift have a substantial sleep duration that meets the recommended duration [6,60–70], regardless of their chronotype. No significant differences were found between genders in regard to the sleeping habits of adolescents with a morning school shift. These results are similar to those reported by Wolfson [75] and Liu et al. [76]. However, previous studies have reported that girls are most frequently experience waking up at night [77], short sleep duration and decreased sleep quality, unlike boys [43,78,79]. An explanation of these tendencies is that they can be associated with an earlier onset of puberty in girls than in boys, which causes changes in sleep schedules [6,80]. Despite this, in the literature there is no consensus regarding gender differences in sleep parameters due to the different methods that have been used in various populations.
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As well as with the school shifts, there were no significant differences in regard to circadian preference and gender. These results are consistent with those reported by previous studies [40,43,81,82]. Nonetheless, others have found that girls tend to be more oriented to eveningness [83,84] or to morningness [85]. Discrepancies in gender differences in chronotypes may be explained as related to age range, cultural aspects, and the interaction between social and biological factors [38]. On the other hand, some studies have indicated that in order to detect gender differences in chronotypes, it is necessary to have a large sample [38,86]. However, the study of gender and age differences in morningness/eveningness in different cultural contexts contributes to a better understanding of these discrepancies [87]. Although not significant, in this study a greater tendency toward eveningness by boys was found, in contrast to the study by Díaz-Morales and Gutierrez [84], in which the sample also was not significant but showed an important tendency according to gender. Our data confirm the results reported by previous studies in terms of individual differences in betime, waking time, and sleep duration that exist among the extreme chronotypes [36,88]. The differences that occur in the duration of sleep in this study, allow us to identify that adolescents who attend classes in a morning shift have sleep deprivation. In addition, adolescents who are oriented to eveningness show greater sleep deprivation. This lack of sleep brings with it a number of sleep problems, such as insomnia, daytime sleepiness, and disrupted sleep patterns [36], with the consequence of unsuitable conditions for school learning [89]. In comparison, adolescents who attend classes it an afternoon shift have better sleeping conditions, regardless of whether they have an orientation to eveningness or morningness, as also shown by Koscec et al. [72]. In Mexico, this afternoon school shift is permanently implemented, so this allows us to compare the morning and afternoon shifts. However, many countries have only a morning school 12
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shift, and recently researchers have indicated that sleep deprivation has been increasing [29,90]. In these countries, the proposal that has emerged to attenuate sleep deprivation is to delay the
Comment [KVS2]: AUTHOR: Carskadon 2002 was cited in the text and numbered reference 90, but was not in the reference list. This work has been added to the reference list as reference 90; please supply full information there.
school schedule. Various studies have delayed the school start time from 20 to 85 minutes, which has shown positive effects on adolescents, including more sleep during the week, decreased daytime sleepiness, increased daily attendance at school, improved school grades, and lower rates of depression and automobile accidents [91,92]. Despite the aforementioned benefits, there is some reluctance to implement changes in school schedules, especially as a result of the various existing policies of schools and teachers [93]. There is also the concern of parents, who have been found to believe that the change in school hours would cause adolescents to have less time to do their homework or engage in extracurricular activities; nevertheless, in the study of [91] adolescents successfully performed these activities despite the delay in school hours. Although parental concerns and educational policies have prevented the implementation of delaying school hours, it would be advisable to change the school start time to later hours, to make it compatible with the circadian rhythms of adolescents, their sleep–wake cycles and their rhythms in cognitive processes [92,94]. Our study provides results regarding the behavior of sleep–wake habits and circadian preferences of Mexican adolescents who have the option to attend classes in either morning or afternoon shifts, as well as the consequences and benefits of these, and contributes to the scarce literature [3]. Nevertheless, there are some limitations. First, sleep habits were self-reported and subjective. Second, our analysis considered sleep time only on school days and weekends, disregarding naptimes. Third, actigraphy would be a more convenient method to evaluate the sleep/wake cycle, although it would be more difficult to use in a large sample. Fourth, we did not consider any kind of social activity that could cause greater impairment of sleep in adolescents. 13
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Finally, a larger sample of adolescents from different parts of Mexico would add to this study and provide a significant contribution to the literature.
5. Conclusion
Our study examined differences in the sleep schedules that exist among Mexican adolescents. We found that adolescents who attended classes in the morning shift were sleep deprived during weekdays, which was further accentuated by circadian preference for eveningness. In contrast, adolescents who attended the afternoon shift tended to have an adequate duration of sleep, regardless of whether they had a circadian preference for eveningness or morningness. The various school schedules worldwide deserve further attention from chronobiologists.
Acknowledgements This project was supported by the National Council of Science and Technology (CONACyT) in Mexico. The authors thank Ms. Pamela Garza for assistance with the administration of questionnaires, the teachers of the José de Escandón secondary school for accommodating the program in their courses, and all students who participated in the study.
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[67] Peixoto C, Teixeira A, Carskadon M, Louzada F. Adolescents living in homes without electric lighting have earlier sleep times. Behav Sleep Med 2009;7:73–80. [68] Carskadon M, Harvey K, Duke P, Anders T, Litt I, Dement W. Pubertal changes in daytime sleepiness. Sleep 1980;2:453–60. [69] Carskadon M, Harvey K, Dement W. Sleep loss in young adolescents. Sleep 1981;4:299– 312. [70] Hirshkowitz M, Whiton K, Albert S, Alessi C, Bruni O, DonCarlos L, et al. National Sleep Foundation´s sleep time duration recommendations: Methodology and results summary. Sleep Health 2015;1:40–3. [71] Olds T, Blunden S, Petkov J, Forchino F. The relationships between sex, age, geography and time in bed in adolescents: A meta-analysis of data from 23 countries. Sleep Med Rev 2001;14:371–8 [72] Koscec A, Radosevic-Vidacek B, Bakotic M. Morningness-eveningness and sleep patterns of adolescents attending school in two rotating shifts. Chronobiol Int 2014;31:52–63. [73] Natal C, Lourenco T, Silva L, Boscolo R, Silvia A, Tufik S, et al. Gender difference in the sleep habits of 11-13 year olds. Rev Brasil Psiquiatr 2009;31:358–61. [74] Lazaratou H, Dikeos D, Anagnostopoulos D, Sbokou O, Soldatos C. Sleep problems in adolescence: A study of senior high school students in Greece. Eur Child Adolesc Psychiatry 2005;14:237–243. [75] Wolfson A. Sleeping patterns of children and adolescents: Developmental trends, disruptions and adaptations. Child Adolesc Psychiatr Clin North Am 1996;5:549–68. 22
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[76] Liu X, Uchiyama M, Okawa M, Kurita H. Prevalence and correlates of self-reported sleep problems among Chinese adolescents. Sleep 2000;23:27–34. [77] Giannotti F, Cortesi F, Ottaviano S. Sleep patterns, daytime functioning and school performance in adolescence. Sleep Res 1997;26:196. [78] Cortesi F, Giannotti F, Caramadre A. Subjective sleep quality among healthy adolescents. Sleep Res 1997;26:193. [79] Laberge L, Petit D, Simard C, VItaro F, Tremblay R, Montplaisir J. Development of sleep patterns in early adolescence. J Sleep Res 2001;10:59–67. [80] Petersen A, Crockett L, Richards M, Boxer A. A self-report measure of pubertal status: Reliability, validity, and initial norms. J Youth Adolesc 1988;17:117–33. [81] Gau S, Soong W. The transition of sleep-wake patterns in early adolescence. Sleep 2003;26:449–54. [82] Kim S, Dueker G, Hasher L, Goldstein D. Children´s time of day preference: Age, gender and ethnic differences. Personal Indiv Diff 2002;33:1083–90. [83] Caci H, Robert P, Dossios C, Boyer P. L'échelle de matinalité pour enfants et adolescents: Propriétés psychométriques et effet du mois de naissance. L'Encéphale 2005;31:56–64. [84] Díaz-Morales J, Gutiérrez M. Morningness-eveningness in adolescents. Spanish J Psychol 2008;11:201–6. [85] Warner S, Murray G, Meyer D. Holiday and school-term sleep patterns of Australian adolescents. J Adolesc 2008;31:595–608.
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[86] Mateo M, Díaz-Morales J, Escribano C, Delgado P, Randler C. Morningness-eveningness and sleep habits among adolescents: Age and gender differences. Psicothema 2012;24:410–5. [87] Schmidt S, Randler C. Morningness-eveningness and eating disorders in a sample of adolescent girls. J Indiv Diff 2010;31:38–45. [88] Tonetti L, Fabbri M, Martoni M, Natale V. Circadian preference and perceived quality of the sleep/wake cycle in Italian high school students. Personal Indiv Diff 2013;54:315–7. [89] Valdez P, Reilly T, Waterhouse J. Rhythms of mental performance. Mind Brain Educ 2008;2:7–16. Comment [KVS6]: AUTHOR: As queried previously, please supply full reference information for reference 90.
[90] Carskadon 2002. [91] Boergers J, Gable C, Owens J. Later school start time is associated with improved sleep and daytime functioning in adolescents. J Dev Behav Pediatr 2014;35:11–7. [92] Valdez P, Ramírez C, García A. Circadian rhythms in cognitive processes: Implications for school learning. Mind Brain Educ 2014;8:161–8. [93] Wahlstrom K. The prickly politics of school starting times. Kappan 1999;80:344–7. [94] Owens J, Belon K, Moss P. Impact of delaying school star time on adolescent sleep, mood,
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and behavior. Arch Pediatr Adolesc Med 2010;164:609–14.
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Fig. 1. Sleep–wake cycle of adolescents from the morning shift and afternoon shift during weekdays (white bars) and weekends (black bars).
Fig. 2. Sleep–wake cycle of adolescents with different chronotypes (evening type, intermediate type, and morning type) during weekdays and weekends.
Fig. 3. Interaction effects of school shift and chronotype on sleep–wake cycle during weekdays (A, C, E) and weekends (B, D, F). Significant interactions were found for waking time (C) and sleep duration (E) on weekdays.
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S1389-9457(15)00856-4 http://dx.doi.org/doi: 10.1016/j.sleep.2015.05.026 SLEEP 2829
To appear in:
Sleep Medicine
Received date: Revised date: Accepted date:
25-3-2015 15-5-2015 29-5-2015
Please cite this article as: Arturo Arrona-Palacios, Aída García, Pablo Valdez, Sleep–wake habits and circadian preference in Mexican secondary school, Sleep Medicine (2015), http://dx.doi.org/doi: 10.1016/j.sleep.2015.05.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Original Article Sleep–wake habits and circadian preference in Mexican secondary school Arturo Arrona-Palaciosa*, Aída Garcíaa, Pablo Valdeza *Corresponding author at: Laboratory of Psychopshysiology, School of Psychology, Universidad Autónoma de Nuevo León, Mutualismo 110, Col Mitras Centro, Monterrey, Nuevo León 64460, México. Tel.: +52 81 83483866. E-mail address: [email protected] (A. Arrona-Palacios).
Highlights
The effects of sleep-wake habits and chronotype on school shifts were explored. Students from the morning shift have short sleep duration. Students from the morning shift with an evening type have shorter sleep duration. Students from the afternoon shift have adequate sleep duration.
ABSTRACT Objective: The current study aimed to determine the differences between sleep–wake habits and circadian preference in Mexican adolescents attending classes at a morning shift or an afternoon shift. Methods: A sample of 568 students of a secondary school in Reynosa, northeastern Mexico, of whom 280 were boys and 288 were girls (mean age 14.08 ± 0.72 years, age range 13–16 years). 1
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In the morning shift, 287 students attend classes on a schedule from 7:30 to 13:00 and the afternoon shift, 281 students, on a schedule from 13:20 to 19:00 Students completed a general information questionnaire, the Sleep Timing Questionnaire and the Spanish version of the Morningness–Eveningness Questionnaire. Results: The adolescents who attended the morning shift had earlier bedtime and waking time, but shorter sleep duration than those who attended the afternoon shift. Those oriented to eveningness had later bedtime, waking time, and a shorter sleep duration than those oriented to morningness. Two interactions were found between school shift and chronotype: first, with regard to waking time during weekdays, students who attended the afternoon shift and were oriented to eveningness woke up later than those who attended the morning shift and were oriented to eveningness; and second, during weekdays, there were no differences between the waking time of morning-type and evening-type students who attended the morning shift. With regard to sleep duration on weekdays, students who attended the morning shift and were oriented to eveningness have the shortest sleep duration. Furthermore, there were no differences between sleep duration on weekdays in evening-type and morning-type students of the afternoon shift. Conclusion: Adolescents who attend classes in the morning shift and are oriented to eveningness are the most sleep deprived. Those who attend the afternoon shift will have optimal sleep duration, regardless of their circadian preference. Keywords: Sleep habits; Adolescents; Chronotype; Sleep education; Mexico; School shifts 1. Introduction During adolescence, the school schedule is the most damaging environmental constraint in sleep habits, especially if is taken into account that the adolescent usually attends school at 07:30 in 2
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the morning throughout the week. On the other hand, a phase delay occurs in the sleep–wake Comment [KVS1]: AUTHOR: Please note that all references have been numbered in order of citation, in keeping with journal style.
cycle during adolescence [1], which makes it difficult to sleep early; and therefore adolescents often go to sleep after midnight. Thus, many adolescents get only 6 or less hours of sleep, thereby causing sleep deprivation [2]. Sleep duration is reduced on weekdays, as adolescents have to get up early to attend classes and tend to stay up late. However, on weekends, holidays, and vacations, adolescents often have longer sleep duration and go to sleep and wake up later [3– 5]. Thus, on weekdays, a sleep debt occurs in adolescents, which causes the appearance of certain sleep disorders such as daytime sleepiness and the delayed sleep phase syndrome, among others [6,7]. Sleep deprivation in adolescents has been associated with a deterioration in general health and immune function [8], increased risk of injuries and accidents [9], increased suicidal ideation [10,11], and increased use of drugs and alcohol [12,13], as well as deficiencies in motor skills [14], working memory [15,16], attention [6,17,18], and retention of information [19,20], with a decline in academic performance [21].. Currently there is a global concern about the increasing prevalence of sleep problems during adolescence [22]. Iglowstein et al. [23] found that the average duration of sleep in adolescents 11 to 16 years old decreased from 9.6 hours to 8.1 hours of sleep. In contrast, other studies suggest that teens are having shorter sleep duration, with an average of 7.6 hours per night [10,24]. In general, the diverse conditions from the social environment have a great influence on the reduction of sleep in adolescents [25,26]. Apart from school schedule, previous studies have indicated that the increasing amount of homework [7,27], lack of parental control [28,29], extracurricular activities [26,30,31], and recently the use of electronic media [32,33] contribute significantly to the adolescent sleeping less. However, one cannot ignore the fact that the need 3
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for sleep is not manifest at the same time in every person. The preference to perform a greater number of activities in the morning or at night is attributed to interindividual differences in the timing of circadian rhythms, which comprise as a continuum between the two extremes [34]. Individuals with a morning-type consitution have no problem with early rising, soon achieve their maximum of mental and physical activity, and become tired early in the evening. People who have an evening type tend to wake up late, become progressively more activate throughout the day, and are most active in the evening or at night; as a consequence, their need for sleep tends to manifest later compared with the that in morning-type individuals. Furthermore, there is also an intermediate type; these individuals fit into an intermediate portion between the two ends of the continuum, and constitute the largest part of the population [35,36]. Adolescents change from morning type to evening type from 12 to 14 years of age accordingly to their own biological changes at this age, as changes in their social environment occur [37]. With regard to gender, boys tend to be more oriented to eveningness than girls [38,39]. Nevertheless, the results of the gender differences in sleep patterns are contradictory. Several studies have found no gender differences at bedtime (weekdays/weekends) and waking time on weekdays [40,41]. Nevertheless, other studies have found that girls wake up earlier on weekdays but later on weekends [7,42], and go to bed earlier on weekdays [40,41,43] than boys. There are also cultural differences, but these are mediated by social, biological, and environmental factors [44,45]. Being an evening type has been identified in adolescents as a risk factor for behavioral and emotional problems such as consumption of alcohol, higher probability of smoking, caffeinated soft drinks, obesity, depression, minor psychiatric disorders, poor school performance and poor perceived quality of sleep [43,46,47,49]. The aforementioned risk factors are due to the divergence that occurs regarding the internal body clock and the calendar of social 4
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activities, especially for those who are evening-type persons. Therefore, these individuals have difficulty waking up early on workdays/weekdays (in this case, to attend classes), and they tend to take advantage of free days to rest and recover the loss of sleep on workdays [46,50]. Despite the cited reports, most data regarding sleep habits and circadian preferences of adolescents are based on research conducted in the United States and several countries in Europe and Oceania. There have been few studies that have examined sleep habits in a Mexican sample [51,52], and fewer with sleep habits of students attending school on different schedules [53]. The current study aimed to determine the differences between sleep–wake habits and circadian preference in Mexican adolescents attending classes during a morning shift or an afternoon shift.
2. Methods 2.1. Participants The final sample consisted of 568 students of a secondary school in Reynosa, northeastern Mexico (280 boys and 288 girls). The mean age of the overall sample was 14.08 ± 0.72 years, and the age range was between 13 and 16 years. In the morning shift, 287 students attended classes on a schedule from 7:30 to 13:00, and in the afternoon shift, 281 students attended classes on a schedule of 13:20 to 19:00. The Mexican secondary school consists of a 3-year program; students are distributed into 18 groups for the morning shift and 18 groups for the afternoon shift. Of the 18 groups, these are divided into six groups per school year for both shifts. For this study, a total sample size of 600 students was selected, which was partitioned into six groups, three for the morning shift and three for the afternoon shift (one group per school year). A total of 32 participants were excluded due to nonresponse or refusal to participate. The application 5
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had an estimated response time of 60 minutes; these questionnaires were applied at the same time during school classes for both shifts. Data collection was voluntary, and students received no payment of any kind. Each participant signed a letter of informed consent, as did the parents of participants who were underage. The research project was approved by the Academic Board of the University, and complied with the principles of the Declaration of Helsinki for research on human participants. 2.2. Measures 2.2.1. General information questionnaire A general information was used to obtain demographic information such as age, gender, school grade, and school shift. 2.2.2. Sleep Timing Questionnaire The Sleep Time Questionnaire is a single-administration instrument that allows determination of an individual’s habitual waking time and bedtime [54]. This questionnaire consists of 18 questions and is divided into two parts: the first section deals with sleeping habits during weekdays (school days), and the second section deals with sleeping habits during the weekend (free days). 2.2.3. Spanish version of the Morningness–Eveningness Questionnaire The Spanish version of the Morningness–Eveningness Questionnaire consists of 19 questions with a four-point Likert scale or a time of day scale. This questionnaire was designed to determine the temporary preference in the activities of individuals (chronotypes), classifying
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them according to their scores as morning types (59–86), intermediate types (42–58), and evening types (16–41) [35,55]. 2.3. Statistical analyses To evaluate differences for each dependent variable, a factorial analysis of variance (ANOVA) was performed, considering three factors, namely, school shift (morning and afternoon), gender, and circadian preference (morning type and evening type), with sleep variables (waking time, bedtime, and sleep duration) as dependent variables. The indicators that showed significant differences in the factorial ANOVA were subjected to a post hoc analysis using the Tukey honest significant difference (HSD) test.
3. Results 3.1. School shift No significant differences in gender were found in any sleep variable. Significant differences were found in the sleep–wake cycle between the morning shift and the afternoon shift, as well as on weekdays and weekends. During the week, participants in the morning shift went to bed earlier (morning shift: 23:00 ± 1:11; afternoon shift: 00:06 ± 1:36; F1,567 = 86.1, p < 0.001) and woke up earlier (morning shift: 06:11 ± 0:29; afternoon shift: 09:29 ± 1:20; F1, 567 = 1537.84, p < 0.001), but their total sleep duration was shorter (morning shift: 7:11 ± 1:13; afternoon shift: 9:23 ± 1:43; F1,567 = 309.83; p < 0.001) compared with participants in the afternoon shift. On weekends, the participants of the morning shift went to bed earlier (morning shift: 00:30 ± 1:51; afternoon shift: 01:12 ± 2:02; F1,567 = 17.60, p < 0.001) and woke up earlier (morning shift:
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10:30 ± 1:47; afternoon shift: 10:58 ± 1:57; F1,567 = 8.82, p < 0.01) compared with the afternoon shift. There was no significant difference in the total of sleep duration (morning shift: 10:00 ± 2:07; afternoon shift: 9:46 ± 2:17, F1,567 = 1.43, not significant) (Fig. 1). 3.2. Circadian preference The morning shift had a total of 47 evening types, 233 intermediate types, and 17 morning types, whereas the afternoon shift had a total of 59 evening types, 208 intermediate types, and 14 morning types. Significant differences between extreme chronotypes were found on weekdays and weekend sleeping habits. During weekdays, participants oriented to eveningness went to bed later (evening type: 24:22 ± 1:36; morning type: 22:32 ± 1:01 h; F2,567 = 25.95, p < 0.001), woke up later (evening type: 08:19 ± 2:12; morning type: 07:17 ± 1:37; F2,567 = 5.80, p < 0.01), and had a shorter sleep duration (evening type: 7:57 ± 2:15 h; morning type: 8:45 ± 1:28 h; F2,567 = 9.478, p < 0.001) than those oriented to morningness. During the weekend, participants who were oriented to eveningness went to bed later (evening type: 01:49 ± 1:56; morning type: 23:01 ± 1:20; F2,567 = 27.28, p < 0.001) and awoke later (evening type: 11:30 ± 1:50; morning type: 9:09 ± 1:42, F2,567 = 59.97, p < 0.001) than those oriented to morningness. No significant differences were found with regard to sleep duration (Fig. 2).
3.3 School shift and circadian preference interactions No significant interactions were found between the school shift and chronotype with bedtime on weekdays (Fig. 3A) and weekend (Fig. 3B). The analysis revealed a significant interaction between the school shift and chronotype with waking time on weekdays (F2,567 = 4.45; p < 0.01) (Fig. 3C). The effect of this interaction is that all the participants who attend classes in the 8
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morning shift must get up early to go to school, whereas those who attend classes in the afternoon shift have the opportunity to wake up at any time due to classes starting at 13:30. In the afternoon shift, the differences in waking time of those who are oriented to eveningness (09:59) and morningness (08:47) differ significantly (p < 0.001) during the weekdays. In contrast, participants from the morning shift who are oriented to eveningness have no significant differences in their waking time from participants oriented to morningness during these days. Furthermore, those who attend the afternoon shift and are oriented to eveningness wake up later (09:59) than those who attend the morning shift and are oriented to eveningness (06:12), with a difference of p < 0.001. No significant interactions were found between school shift and chronotype with regard to waking time on the weekend (Fig. 3D). In addition, a significant interaction between school shift and chronotype on sleep duration of weekdays was found (F2,567 = 4.499; p < 0.01) (Fig. 3E). This interaction is due to the fact that participants in the morning shift who are oriented to eveningness go to sleep later, but wake up early to attend classes, thus causing shorter sleep duration (6:16) compared to participants who are oriented to morningness (7:53), differing significantly (p < 0.001) during weekdays. In contrast, participants from the afternoon shift who are oriented to eveningness have no significant differences in their sleep duration from participants oriented to morningness. Furthermore, those who attend the morning shift and are oriented to eveningness have shorter sleep duration (6:16) than those who attend the afternoon shift and are oriented to eveningness (9:17) with a difference of p < 0.001. No significant differences were found between the school shift and chronotype with sleep duration on weekend (Fig. 3F).
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4. Discussion
This report provides information on sleep–wake habits and circadian preference among adolescents of two different school shifts—morning and afternoon shifts—in a Mexican population, which has rarely been examined both nationally and internationally [3]. Our main finding was that adolescents who attend classes in a morning shift tend to sleep less compared with those who attend classes in an afternoon shift. Our results also showed that adolescents who have a circadian preference for eveningness tend to sleep and wake up later, regardless of shift and gender. Those who have a circadian preference for eveningness and attend classes in the morning shift have the shortest sleep duration. In our study, adolescents who attended classes in the morning shift, slept during the week an average of 7 hours 11 minutes. When comparing the sleep habits of Mexican adolescents in this study to those of adolescents worldwide, Mexican adolescents slept about the same amount of hours as adolescents in Saudi Arabia [56,57], South Africa [58], Greece [59], Israel [60], and Taiwan [61]. On the other hand, Mexican adolescents slept fewer hours than reported in adolescents from different countries such as Spain [62], Australia [63], Germany [64], United States [65], China [66], and Brazil [67] where adolescents sleep approximately more than 8 hours. This suggests that adolescents attending classes in the morning shift have a duration of sleep below what is recommended (8.5–9.25 hours) [6,68,69]. The National Sleep Foundation [70] classifies sleep duration of adolescents into three levels as follows: recommended (8–10 hours), may be appropriate (7–11 hours), and not recommended (less than 7 and more than 11 hours); thus, the sleep duration of morning shift students is close to the inferior limit that may be
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appropriate, whereas students with an evening chronotype sleep and average of 6 hours and 16 minutes, making it a nonrecommended sleep duration for these students. These comparisons show the cultural differences in adolescents’ duration of sleep, as demonstrated in reviews by Gradisar et al. [3] and Olds et al. [71]. In addition to these cultural differences with adolescents who attend classes at a morning shift, there are differences also in the sleep habits of those who attend an afternoon shift, which has been only slightly explored because, in most countries, this type of school system (with a morning and afternoon shift) is not implemented. Adolescents who attended classes in an afternoon shift slept an average of 9 hours and 23 minutes during the week, which is slightly higher than has been reported in countries where the school system has an afternoon shift such as Croatia [72], Brazil [67,73], Greece [74], and even in an earlier study in Mexico [53] which report an average weekday sleep duration of 8 to 8.30 hours. Thus adolescents who have the opportunity to attend classes in an afternoon shift have a substantial sleep duration that meets the recommended duration [6,60–70], regardless of their chronotype. No significant differences were found between genders in regard to the sleeping habits of adolescents with a morning school shift. These results are similar to those reported by Wolfson [75] and Liu et al. [76]. However, previous studies have reported that girls are most frequently experience waking up at night [77], short sleep duration and decreased sleep quality, unlike boys [43,78,79]. An explanation of these tendencies is that they can be associated with an earlier onset of puberty in girls than in boys, which causes changes in sleep schedules [6,80]. Despite this, in the literature there is no consensus regarding gender differences in sleep parameters due to the different methods that have been used in various populations.
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As well as with the school shifts, there were no significant differences in regard to circadian preference and gender. These results are consistent with those reported by previous studies [40,43,81,82]. Nonetheless, others have found that girls tend to be more oriented to eveningness [83,84] or to morningness [85]. Discrepancies in gender differences in chronotypes may be explained as related to age range, cultural aspects, and the interaction between social and biological factors [38]. On the other hand, some studies have indicated that in order to detect gender differences in chronotypes, it is necessary to have a large sample [38,86]. However, the study of gender and age differences in morningness/eveningness in different cultural contexts contributes to a better understanding of these discrepancies [87]. Although not significant, in this study a greater tendency toward eveningness by boys was found, in contrast to the study by Díaz-Morales and Gutierrez [84], in which the sample also was not significant but showed an important tendency according to gender. Our data confirm the results reported by previous studies in terms of individual differences in betime, waking time, and sleep duration that exist among the extreme chronotypes [36,88]. The differences that occur in the duration of sleep in this study, allow us to identify that adolescents who attend classes in a morning shift have sleep deprivation. In addition, adolescents who are oriented to eveningness show greater sleep deprivation. This lack of sleep brings with it a number of sleep problems, such as insomnia, daytime sleepiness, and disrupted sleep patterns [36], with the consequence of unsuitable conditions for school learning [89]. In comparison, adolescents who attend classes it an afternoon shift have better sleeping conditions, regardless of whether they have an orientation to eveningness or morningness, as also shown by Koscec et al. [72]. In Mexico, this afternoon school shift is permanently implemented, so this allows us to compare the morning and afternoon shifts. However, many countries have only a morning school 12
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shift, and recently researchers have indicated that sleep deprivation has been increasing [29,90]. In these countries, the proposal that has emerged to attenuate sleep deprivation is to delay the
Comment [KVS2]: AUTHOR: Carskadon 2002 was cited in the text and numbered reference 90, but was not in the reference list. This work has been added to the reference list as reference 90; please supply full information there.
school schedule. Various studies have delayed the school start time from 20 to 85 minutes, which has shown positive effects on adolescents, including more sleep during the week, decreased daytime sleepiness, increased daily attendance at school, improved school grades, and lower rates of depression and automobile accidents [91,92]. Despite the aforementioned benefits, there is some reluctance to implement changes in school schedules, especially as a result of the various existing policies of schools and teachers [93]. There is also the concern of parents, who have been found to believe that the change in school hours would cause adolescents to have less time to do their homework or engage in extracurricular activities; nevertheless, in the study of [91] adolescents successfully performed these activities despite the delay in school hours. Although parental concerns and educational policies have prevented the implementation of delaying school hours, it would be advisable to change the school start time to later hours, to make it compatible with the circadian rhythms of adolescents, their sleep–wake cycles and their rhythms in cognitive processes [92,94]. Our study provides results regarding the behavior of sleep–wake habits and circadian preferences of Mexican adolescents who have the option to attend classes in either morning or afternoon shifts, as well as the consequences and benefits of these, and contributes to the scarce literature [3]. Nevertheless, there are some limitations. First, sleep habits were self-reported and subjective. Second, our analysis considered sleep time only on school days and weekends, disregarding naptimes. Third, actigraphy would be a more convenient method to evaluate the sleep/wake cycle, although it would be more difficult to use in a large sample. Fourth, we did not consider any kind of social activity that could cause greater impairment of sleep in adolescents. 13
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Finally, a larger sample of adolescents from different parts of Mexico would add to this study and provide a significant contribution to the literature.
5. Conclusion
Our study examined differences in the sleep schedules that exist among Mexican adolescents. We found that adolescents who attended classes in the morning shift were sleep deprived during weekdays, which was further accentuated by circadian preference for eveningness. In contrast, adolescents who attended the afternoon shift tended to have an adequate duration of sleep, regardless of whether they had a circadian preference for eveningness or morningness. The various school schedules worldwide deserve further attention from chronobiologists.
Acknowledgements This project was supported by the National Council of Science and Technology (CONACyT) in Mexico. The authors thank Ms. Pamela Garza for assistance with the administration of questionnaires, the teachers of the José de Escandón secondary school for accommodating the program in their courses, and all students who participated in the study.
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Fig. 1. Sleep–wake cycle of adolescents from the morning shift and afternoon shift during weekdays (white bars) and weekends (black bars).
Fig. 2. Sleep–wake cycle of adolescents with different chronotypes (evening type, intermediate type, and morning type) during weekdays and weekends.
Fig. 3. Interaction effects of school shift and chronotype on sleep–wake cycle during weekdays (A, C, E) and weekends (B, D, F). Significant interactions were found for waking time (C) and sleep duration (E) on weekdays.
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