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Sleep extension improves serving accuracy: A study with college varsity tennis players
Physiology & Behavior 151 (2015) 541–544
Contents lists available at ScienceDirect
Physiology & Behavior journal homepage: www.elsevier.com/locate/phb
Sleep extension improves serving accuracy: A study with college varsity tennis players Jennifer Schwartz a,⁎, Richard D. Simon Jr a b c
b,c
School of Population and Public Health, University of British Columbia, Vancouver, British Columbia, Canada Kathryn Severyns Dement Sleep Disorders Center, Walla Walla, WA, United States University of Washington, Seattle, WA, United States
a r t i c l e
i n f o
Article history: Received 10 April 2015 Accepted 26 August 2015 Available online xxxx Keywords: Sleep extension Tennis Serving accuracy Sleep deprivation
a b s t r a c t Objective: This study investigated the effects of sleep extension on tennis serving accuracy, as well as daytime sleepiness in college varsity tennis players. Methods: Twelve (seven females and five males) healthy students on a college varsity tennis team maintained their habitual sleep–wake schedule for a one-week baseline period followed by a one-week sleep extension period. Participants were requested to sleep at least nine hours, including naps, during the sleep extension period. Serving accuracy was assessed when participants were sleep deprived (prior to the sleep extension period) and after the sleep extension period. Levels of daytime sleepiness were monitored via the Epworth Sleepiness Scale and the Stanford Sleepiness Scale, and caffeine consumption was recorded throughout the study. Results: Participants slept significantly more in the second week — the sleep extension week — compared with the first week — the baseline week (8.85 vs. 7.14 h; p b 0.05). Following the sleep extension period, accuracy of the tennis serves improved significantly (35.7% vs. 41.8%; p b 0.05), and the Epworth Sleepiness Scale and Stanford Sleepiness Scale scores declined significantly (12.15 vs. 5.67; p b 0.05 and 3.56 vs. 2.67; p b 0.05, respectively). Conclusions: This study demonstrates that an increase in sleep of approximately 2 h per night significantly increased athletic performance in college varsity tennis players. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Sleep is an indispensable part of human life; the fact that humans sleep for approximately one-third of the time is an index of its importance for healthy physiology. Sleep deprivation has been shown to negatively affect psychological factors, mental and physical performance, alertness, reaction time, capacity for prolonged exercise, and memory [1–3]. Additionally, inadequate sleep is associated with an increased risk of several conditions, including coronary heart disease, obesity, diabetes, and hypertension [4–7]. College students in the United States generally experience significant levels of sleep deprivation, sleep disturbances, and daytime sleepiness, perhaps due to their notoriously irregular sleep schedules. In particular, college students often experience social jet lag — a term for circadian disruption or misalignment between the biological clock and social schedule resulting from temptations and demands of college life [8]. For example, college students in a large (n = 1823) study by Orzech et al. reported an average of 6 h and 41 min of sleep per night, while the
⁎ Corresponding author. E-mail address: [email protected] (J. Schwartz).
http://dx.doi.org/10.1016/j.physbeh.2015.08.035 0031-9384/© 2015 Elsevier Inc. All rights reserved.
National Sleep Foundation recommends 7 to 9 h of sleep per night for adults over the age of 18 [9,10]. Additionally, depending upon the cohort analyzed, between 25% and 50% of college students report substantial daytime sleepiness [11–13]. Therefore, these high levels of sleep deprivation and poor-quality sleep in college students potentially provide greater opportunity for inadequate sleep to impact health. The tennis serve is an ideal model of a complex sensorimotor task, as it is both cognitively and physically challenging. It is a behavior that is sensitive to multiple factors because it requires concentration, motivation, balance, alertness, coordination, motor learning and memory, strength, and perceptual memory. Virtually every area of the brain contains neurons that are active during the performance of a tennis serve, and these diverse regions must work together in order to achieve a desirable outcome. For example, the visual system, innumerable regions of the cerebral cortex, the reticular formation, the amygdala, the hypothalamus, the corpus striatum, and the basal ganglia are a few of the brain regions that are activated throughout the execution of the tennis serve [14]. Therefore, the serve is most likely affected by the amount of sleep one gets and should clearly demonstrate the impacts of sleep on countless facets of the human body. Extended sleep has been shown to benefit reaction times, mood, alertness, perceptual and motor memory consolidation and recall, the
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J. Schwartz, R.D. Simon Jr / Physiology & Behavior 151 (2015) 541–544
learning of motor skills, and general performance [15–18]. Nevertheless, the consequences of sleep extension on complex, integrative motor functions have been studied far less than the effects of sleep deprivation on such functions. Therefore, the current study aims to investigate the effects of sleep extension on the tennis serve in college varsity tennis players, a group who practices their serves on a daily basis, thus being less likely to benefit from one extra practice session of such a routine task. 2. Methods 2.1. Participants Seven female and five male members of the women's and men's varsity tennis teams at a liberal arts NCAA Division III college in Walla Walla, Washington agreed to participate in this study. They were between the ages of eighteen and 22, with a mean age of 20.2 years old. Participants were informed that this study would require them to sleep for 9 h every night for seven days. Participants were paid $50 to participate in the study by one of the authors (RDS). The study was approved by the Whitman College Institutional Review Board.
Fig. 2. Tennis serving accuracy at week 1 (post-habitual sleep period) and week 2 (postsleep extension period).
of the study, participants completed the Stanford Sleepiness Scale to rate their feelings of tiredness [19].
2.2. Assessment of sleep deprivation Participants recorded their normal sleep patterns and hours of sleep each night, including naps, for one week (seven days) in a standardized sleep diary. At the beginning of the seven days, an Epworth Sleepiness Scale was administered to all participants, and each day, for the duration
2.3. Design of tennis serve assessment At the end of this first week, all twelve participants served 25 tennis balls into the deuce side of the tennis court and 25 into the add side of the court after warming up for 5 min. Participants were asked to use their “second serves” and were told to aim to hit their serve deep and to the lateral far corner of the service box into a circular target that had a six-foot diameter (Fig. 1). The number of serves that were hit into the circular target was recorded out of the total 50 serves that were hit. The Epworth Sleepiness Scale was administered for the second time to all participants on this day. The two serving sessions took place in an indoor tennis center to assure that wind and sun did not interfere with the participants' serves. Both sessions were performed between four and five pm. 2.4. Regimen of sleep extension During week 2, which began the day after the first serving assessment, participants were asked to begin restoring their sleep by obtaining at least 9 h every night, including naps, for seven days. They recorded the number of hours slept each night, and rated their feelings of tiredness with the Stanford Sleepiness Scale every day for the duration of the study.
2.5. Second tennis serve assessment At the end of the seven days of sleep extension, the participants served a total of 50 tennis balls exactly as they had before their sleep was extended, after warming up for 5 min. Participants recorded the number of balls that were hit into the target out of 50 once again. The Epworth Sleepiness Scale was administered on this day for the third time.
2.6. Caffeine and alcohol consumption
Fig. 1. Aerial view of experimental design of a study on sleep extension and tennis serving accuracy.
Each day participants recorded the number of beverages consumed that contained caffeine. The players were asked not to consume caffeine the day that the serves were performed or alcohol 48 h before the serves were performed in both serving sessions. They were also asked not to nap 2 h before both serving sessions.
J. Schwartz, R.D. Simon Jr / Physiology & Behavior 151 (2015) 541–544
543
sleep significantly more in the second week compared with the first (t(11) = −9.01, p b 0.05). 3.3. Approximately 2 h of sleep extension significantly improved sleepiness Two independent measures both confirmed that the tennis players' feelings of sleepiness significantly decreased from the sleep deprived week to the sleep extension week. As shown in Table 1, a correlated groups t test revealed that the average Stanford Sleepiness Scale scores of the sleep deprived week were significantly higher than those of the sleep extension week (t(11) = 4.94, p b 0.05). As shown in Fig. 4, the average Epworth Sleepiness Scale scores of both the week prior to starting the study and the sleep deprived week, were compared to that of the sleep extension week. A repeated measures analysis of variance confirmed that the Epworth Sleepiness Scale scores also decreased significantly from the first week to the second week (F(2,22) = 19.10, p b 0.01). See Table 1 for a comparison of the means. Fig. 3. Change in serving accuracy of each participant at week 2 (post-sleep extension period).
2.7. Statistical analysis Multiple t tests and a repeated measures analysis of variance were performed using the statistical package SPSS. Since there were no significant gender differences in either serving accuracy, subjective sleepiness measures, or amount of sleep obtained, data for males and females were combined and tested together in all analyses. 3. Results 3.1. Sleep extension improves serving accuracy This study compared performance on a complex sensorimotor task during periods of predicted sleep deprivation and periods of predicted sleep extension. A correlated groups t test was used to compare the mean number of accurate serves at the end of the sleep deprived week with the mean number of accurate serves at the end of the sleep extension week (t(11) = −1.97, p b 0.05, one-tailed). The accuracy of the athletes' tennis serves improved significantly after sleep extension for 7 days (Fig. 2). Fig. 3 shows the percent change (improvement or decline) in the serving accuracy from the sleep deprived week to the sleep extension week of each individual player. 3.2. Confirmation of sleep status In order to confirm that the tennis players were in a sleep deprived state during week 1 of the study, hours of sleep obtained every night/ day, including naps, were reported for the duration of that week. The players were then asked to extend their sleep by obtaining at least 9 h every night/day during the second week, in an attempt to decrease the players' sleep debt. Table 1 shows the mean number of hours of sleep obtained in the first week and in the second week during sleep extension. A paired samples t test indicated that the players did indeed
3.4. Caffeine intake The amount of caffeine consumed during the sleep deprived week was compared with that of the sleep extension week (Table 1). A correlated groups t test revealed no significant difference between the amount of caffeine consumed during the first week and the amount consumed during the second week (t(11) = −0.20, p = 0.41), thereby eliminating caffeine as a potential confounding variable. 4. Discussion Results of this study demonstrate that increases (slightly less than 2 h for seven days/nights) in sleep duration yield significant benefits in accuracy and performance of varsity tennis players' serves. Participants obtained significantly more sleep during the sleep extension period (week 2) than during the sleep deprived period (week 1; habitual sleep). This regimen of sleep extension also had broad positive affects on self-reported wakefulness, as measured by both the Stanford Sleepiness Scale and the Epworth Sleepiness Scale. The fact that there were no significant gender differences in either serving accuracy or wakefulness measures could further reflect the central importance of sleep in preparing the body for optimum performance. In addition, there was no significant difference between the amount of caffeine consumed during the sleep deprived week and the sleep extension week. The tennis serve is an ideal model of a complex, integrative, sensorimotor skill that requires concentration, coordination, perceptual memory, motivation, balance, strength, and the activation and synchronization of multiple neural systems. Therefore, sleep extension could have positively affected the accuracy of the serve by enhancing a number of processes that are necessary to achieve an optimal serve. Nine out of the twelve participants increased their serving accuracy after the period of sleep extension, while the serving accuracy of three of the players decreased. The significant increase in hours of sleep correlated with a significant improvement in the accuracy of the tennis players' serves, however it is important to address alternative explanations. It is improbable
Table 1 Characteristics of participants' tennis serving accuracy, sleep, and caffeine consumption before and after sleep extension.
Accurate serves out of 50 (deuce & add side together), mean (SD) Accurate serves on the deuce side (out of 25), mean (SD) Accurate serves on the add side (out of 25), mean (SD) Sleep per night (hours), mean (SD) Stanford Sleepiness Scale score, mean (SD) Epworth Sleepiness Scale score, mean (SD) Caffeinated beverages consumed, mean (SD)
Week 1 (before sleep extension)
Week 2 (after sleep extension)
p value
17.83 (7.84) 9.25 (5.28) 8.58 (3.32) 7.14 (0.87) 3.56 (0.77) 12.15 (4.20) 0.89 (0.47)
20.92 (6.80) 10.67 (3.99) 10.25 (3.49) 8.85 (0.60) 2.67 (0.64) 5.67 (2.46) 0.94 (0.77)
b0.05
b0.05 b0.05 b0.01 0.41
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J. Schwartz, R.D. Simon Jr / Physiology & Behavior 151 (2015) 541–544
in varsity tennis players' second serves. These results indicating that sleep enhances athletic performance add to previous research revealing that sleep is essential for the formation of motor and perceptual skill memories. Experimental studies with larger samples and objective sleep measures that aim to extend and/or improve sleep are warranted to examine whether performance is enhanced in college-aged students, and in turn, whether downstream harmful outcomes associated with sleep deprivation are reduced. This study also suggests that more research on the effects of sleep extension, rather than sleep restriction, could be informative in college age subjects. Sleep is necessary for optimal performance of motor skills, and it may be necessary before achieving the maximum improvement from practice. Sleep should be an integral part of athletic training programs, and coaches should emphasize the importance of sleep.
References Fig. 4. Epworth Sleepiness Scale scores at baseline, after week 1 (post-habitual sleep period), and after week 2 (post-sleep extension period).
that the increase in accuracy of the serves was due to learning or practice from the first serving session. The participants in this study were college varsity tennis players who practice their serves on a daily basis, so it is unlikely that one extra practice session during this study would improve such a routine task. If the participants had not increased sleep between the two serving sessions, it is unlikely that the accuracy of their serves would have increased appreciably [16]. In fact, previous results reveal that the brain structures that are activated during the learning or training of a serial motor task must be reactivated in order for these motor memories to be consolidated, and the reactivation of these brain structures is most apparent during REM sleep [16]. The Epworth Sleepiness Scale scores were very similar when administered one week prior to starting the study and one week into it, after the players had followed their normal sleeping habits for one week (sleep deprived week). However, their reported feelings of sleepiness then significantly decreased after the period of sleep extension. Therefore, these athletes did indeed feel better rested than they had in at least two weeks, which certainly could have contributed to the increase in accuracy of their serves. This is consistent with many previous studies. For example, Matsumoto and colleagues found that feelings of tiredness negatively correlated with accuracy and performance [20]. The ratings of the Stanford Sleepiness Scale further confirmed this decrease in feelings of sleepiness from the sleep deprived to the sleep extension week. 5. Limitations Several limitations should be mentioned. First, typical collegiate tennis teams include 16–24 male and female members, which limited the number of potential participants that could be enrolled in the study, thereby resulting in a small sample size of 12. Nonetheless, this study included a substantial percentage of the tennis team. Second, measures of sleep duration were based on self-report, which may not be representative of habitual sleep and could have resulted in misclassification bias [21]. Nonetheless, data indicate moderate correlations between selfreported sleep hours and actigraphy (Pearson's r = 0.53) [22]. 6. Conclusions Sleep extension significantly improved the accuracy of tennis serves in the majority of these varsity tennis players. The results of this study suggest that extending sleep in college aged people to the recommended 9 h has the potential to improve the accuracy of a complicated multifaceted task — serving in tennis. Specifically, the current findings demonstrate that sleep extension from a baseline of approximately 7 h per night to nearly 9 h per night resulted in significant improvement
[1] B.J. Martin, Effect of sleep deprivation on tolerance of prolonged exercise, Eur. J. Appl. Physiol. 47 (4) (1981) 345–354. [2] T. Reilly, M. Piercy, The effect of partial sleep deprivation on weight-lifting performance, Ergonomics 37 (1) (1994) 107–115, http://dx.doi.org/10.1080/00140139408963628. [3] C.D. Mah, K.E. Mah, E.J. Kezirian, W.C. Dement, The effects of sleep extension on the athletic performance of collegiate basketball players, Sleep 34 (7) (2011) 943–950, http://dx.doi.org/10.5665/SLEEP.1132. [4] R. Wolk, A.S. Gami, A. Garcia-Touchard, V.K. Somers, Sleep and cardiovascular disease, Curr. Probl. Cardiol. 30 (12) (2005) 625–662, http://dx.doi.org/10.1016/j. cpcardiol.2005.07.002. [5] S.M. Schmid, M. Hallschmid, B. Schultes, The metabolic burden of sleep loss, Lancet Diabetes Endocrinol. (2014)http://dx.doi.org/10.1016/S2213-8587(14)70012-9. [6] F.P. Cappuccio, L. D'Elia, P. Strazzullo, M.A. Miller, Sleep duration and all-cause mortality: a systematic review and meta-analysis of prospective studies, Sleep 33 (5) (2010) 585–592. [7] J. Schwartz, M. Allison, S. Ancoli-Israel, et al., Napping and less disturbed nighttime sleep associated with reduced carotid intima-media thickness in elderly Alzheimer caregivers, The Open Sleep Journal 5 (2012) 43–50, http://dx.doi.org/10.2174/ 1874620901205010043. [8] M. Wittmann, J. Dinich, M. Merrow, T. Roenneberg, Social jetlag: misalignment of biological and social time, Chronobiol. Int. 23 (1–2) (2006) 497–509, http://dx.doi. org/10.1080/07420520500545979. [9] K.M. Orzech, D.B. Salafsky, L.A. Hamilton, The state of sleep among college students at a large public university, J. Am. Coll. Health J. ACH 59 (7) (2011) 612–619, http:// dx.doi.org/10.1080/07448481.2010.520051. [10] National Sleep Foundation (NSF), How much sleep do we really need? Natl. Sleep Found. (2009) http://www.sleepfoundation.org/article/how-sleep-works/howmuch-sleep-do-we-really-need (Accessed August 7, 2012). [11] American College Health Association, National College Health Assessment Fall 2009 Reference Group Report, 2009 http://www.acha-ncha.org (Accessed August 7, 2012). [12] W.C. Buboltz Jr., F. Brown, B. Soper, Sleep habits and patterns of college students: a preliminary study, J. Am. Coll. Health J. ACH 50 (3) (2001) 131–135, http://dx.doi. org/10.1080/07448480109596017. [13] H.G. Lund, B.D. Reider, A.B. Whiting, J.R. Prichard, Sleep patterns and predictors of disturbed sleep in a large population of college students, J. Adolesc. Health. Off Publ. Soc. Adolesc. Med. 46 (2) (2010) 124–132, http://dx.doi.org/10.1016/j. jadohealth.2009.06.016. [14] D. Purves, G.J. Augustine, D. Fitzpatrick, et al., Neuroscience, Third edition Sinauer Associates, Inc, Sunderland, Ma, 2004. [15] B.B. Kamdar, K.A. Kaplan, E.J. Kezirian, W.C. Dement, The impact of extended sleep on daytime alertness, vigilance, and mood, Sleep Med. 5 (5) (2004) 441–448, http://dx.doi.org/10.1016/j.sleep.2004.05.003. [16] S. Fischer, M. Hallschmid, A.L. Elsner, J. Born, Sleep forms memory for finger skills, Proc. Natl. Acad. Sci. U. S. A. 99 (18) (2002) 11987–11991, http://dx.doi.org/10. 1073/pnas.182178199. [17] M. Atienza, J.L. Cantero, R. Stickgold, Posttraining sleep enhances automaticity in perceptual discrimination, J. Cogn. Neurosci. 16 (1) (2004) 53–64, http://dx.doi. org/10.1162/089892904322755557. [18] M.P. Walker, T. Brakefield, A. Morgan, J.A. Hobson, R. Stickgold, Practice with sleep makes perfect: sleep-dependent motor skill learning, Neuron 35 (1) (2002) 205–211. [19] E. Hoddes, V. Zarcone, H. Smythe, R. Phillips, W.C. Dement, Quantification of sleepiness: a new approach, Psychophysiology 10 (4) (1973) 431–436. [20] Y. Matsumoto, K. Mishima, K. Satoh, T. Shimizu, Y. Hishikawa, Physical activity increases the dissociation between subjective sleepiness and objective performance levels during extended wakefulness in human, Neurosci. Lett. 326 (2) (2002) 133–136. [21] D.S. Lauderdale, K.L. Knutson, L.L. Yan, K. Liu, P.J. Rathouz, Self-reported and measured sleep duration: how similar are they? Epidemiol. Camb. Mass. 19 (6) (2008) 838–845, http://dx.doi.org/10.1097/EDE.0b013e318187a7b0. [22] A.R. Wolfson, M.A. Carskadon, C. Acebo, et al., Evidence for the validity of a sleep habits survey for adolescents, Sleep 26 (2) (2003) 213–216.
Contents lists available at ScienceDirect
Physiology & Behavior journal homepage: www.elsevier.com/locate/phb
Sleep extension improves serving accuracy: A study with college varsity tennis players Jennifer Schwartz a,⁎, Richard D. Simon Jr a b c
b,c
School of Population and Public Health, University of British Columbia, Vancouver, British Columbia, Canada Kathryn Severyns Dement Sleep Disorders Center, Walla Walla, WA, United States University of Washington, Seattle, WA, United States
a r t i c l e
i n f o
Article history: Received 10 April 2015 Accepted 26 August 2015 Available online xxxx Keywords: Sleep extension Tennis Serving accuracy Sleep deprivation
a b s t r a c t Objective: This study investigated the effects of sleep extension on tennis serving accuracy, as well as daytime sleepiness in college varsity tennis players. Methods: Twelve (seven females and five males) healthy students on a college varsity tennis team maintained their habitual sleep–wake schedule for a one-week baseline period followed by a one-week sleep extension period. Participants were requested to sleep at least nine hours, including naps, during the sleep extension period. Serving accuracy was assessed when participants were sleep deprived (prior to the sleep extension period) and after the sleep extension period. Levels of daytime sleepiness were monitored via the Epworth Sleepiness Scale and the Stanford Sleepiness Scale, and caffeine consumption was recorded throughout the study. Results: Participants slept significantly more in the second week — the sleep extension week — compared with the first week — the baseline week (8.85 vs. 7.14 h; p b 0.05). Following the sleep extension period, accuracy of the tennis serves improved significantly (35.7% vs. 41.8%; p b 0.05), and the Epworth Sleepiness Scale and Stanford Sleepiness Scale scores declined significantly (12.15 vs. 5.67; p b 0.05 and 3.56 vs. 2.67; p b 0.05, respectively). Conclusions: This study demonstrates that an increase in sleep of approximately 2 h per night significantly increased athletic performance in college varsity tennis players. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Sleep is an indispensable part of human life; the fact that humans sleep for approximately one-third of the time is an index of its importance for healthy physiology. Sleep deprivation has been shown to negatively affect psychological factors, mental and physical performance, alertness, reaction time, capacity for prolonged exercise, and memory [1–3]. Additionally, inadequate sleep is associated with an increased risk of several conditions, including coronary heart disease, obesity, diabetes, and hypertension [4–7]. College students in the United States generally experience significant levels of sleep deprivation, sleep disturbances, and daytime sleepiness, perhaps due to their notoriously irregular sleep schedules. In particular, college students often experience social jet lag — a term for circadian disruption or misalignment between the biological clock and social schedule resulting from temptations and demands of college life [8]. For example, college students in a large (n = 1823) study by Orzech et al. reported an average of 6 h and 41 min of sleep per night, while the
⁎ Corresponding author. E-mail address: [email protected] (J. Schwartz).
http://dx.doi.org/10.1016/j.physbeh.2015.08.035 0031-9384/© 2015 Elsevier Inc. All rights reserved.
National Sleep Foundation recommends 7 to 9 h of sleep per night for adults over the age of 18 [9,10]. Additionally, depending upon the cohort analyzed, between 25% and 50% of college students report substantial daytime sleepiness [11–13]. Therefore, these high levels of sleep deprivation and poor-quality sleep in college students potentially provide greater opportunity for inadequate sleep to impact health. The tennis serve is an ideal model of a complex sensorimotor task, as it is both cognitively and physically challenging. It is a behavior that is sensitive to multiple factors because it requires concentration, motivation, balance, alertness, coordination, motor learning and memory, strength, and perceptual memory. Virtually every area of the brain contains neurons that are active during the performance of a tennis serve, and these diverse regions must work together in order to achieve a desirable outcome. For example, the visual system, innumerable regions of the cerebral cortex, the reticular formation, the amygdala, the hypothalamus, the corpus striatum, and the basal ganglia are a few of the brain regions that are activated throughout the execution of the tennis serve [14]. Therefore, the serve is most likely affected by the amount of sleep one gets and should clearly demonstrate the impacts of sleep on countless facets of the human body. Extended sleep has been shown to benefit reaction times, mood, alertness, perceptual and motor memory consolidation and recall, the
542
J. Schwartz, R.D. Simon Jr / Physiology & Behavior 151 (2015) 541–544
learning of motor skills, and general performance [15–18]. Nevertheless, the consequences of sleep extension on complex, integrative motor functions have been studied far less than the effects of sleep deprivation on such functions. Therefore, the current study aims to investigate the effects of sleep extension on the tennis serve in college varsity tennis players, a group who practices their serves on a daily basis, thus being less likely to benefit from one extra practice session of such a routine task. 2. Methods 2.1. Participants Seven female and five male members of the women's and men's varsity tennis teams at a liberal arts NCAA Division III college in Walla Walla, Washington agreed to participate in this study. They were between the ages of eighteen and 22, with a mean age of 20.2 years old. Participants were informed that this study would require them to sleep for 9 h every night for seven days. Participants were paid $50 to participate in the study by one of the authors (RDS). The study was approved by the Whitman College Institutional Review Board.
Fig. 2. Tennis serving accuracy at week 1 (post-habitual sleep period) and week 2 (postsleep extension period).
of the study, participants completed the Stanford Sleepiness Scale to rate their feelings of tiredness [19].
2.2. Assessment of sleep deprivation Participants recorded their normal sleep patterns and hours of sleep each night, including naps, for one week (seven days) in a standardized sleep diary. At the beginning of the seven days, an Epworth Sleepiness Scale was administered to all participants, and each day, for the duration
2.3. Design of tennis serve assessment At the end of this first week, all twelve participants served 25 tennis balls into the deuce side of the tennis court and 25 into the add side of the court after warming up for 5 min. Participants were asked to use their “second serves” and were told to aim to hit their serve deep and to the lateral far corner of the service box into a circular target that had a six-foot diameter (Fig. 1). The number of serves that were hit into the circular target was recorded out of the total 50 serves that were hit. The Epworth Sleepiness Scale was administered for the second time to all participants on this day. The two serving sessions took place in an indoor tennis center to assure that wind and sun did not interfere with the participants' serves. Both sessions were performed between four and five pm. 2.4. Regimen of sleep extension During week 2, which began the day after the first serving assessment, participants were asked to begin restoring their sleep by obtaining at least 9 h every night, including naps, for seven days. They recorded the number of hours slept each night, and rated their feelings of tiredness with the Stanford Sleepiness Scale every day for the duration of the study.
2.5. Second tennis serve assessment At the end of the seven days of sleep extension, the participants served a total of 50 tennis balls exactly as they had before their sleep was extended, after warming up for 5 min. Participants recorded the number of balls that were hit into the target out of 50 once again. The Epworth Sleepiness Scale was administered on this day for the third time.
2.6. Caffeine and alcohol consumption
Fig. 1. Aerial view of experimental design of a study on sleep extension and tennis serving accuracy.
Each day participants recorded the number of beverages consumed that contained caffeine. The players were asked not to consume caffeine the day that the serves were performed or alcohol 48 h before the serves were performed in both serving sessions. They were also asked not to nap 2 h before both serving sessions.
J. Schwartz, R.D. Simon Jr / Physiology & Behavior 151 (2015) 541–544
543
sleep significantly more in the second week compared with the first (t(11) = −9.01, p b 0.05). 3.3. Approximately 2 h of sleep extension significantly improved sleepiness Two independent measures both confirmed that the tennis players' feelings of sleepiness significantly decreased from the sleep deprived week to the sleep extension week. As shown in Table 1, a correlated groups t test revealed that the average Stanford Sleepiness Scale scores of the sleep deprived week were significantly higher than those of the sleep extension week (t(11) = 4.94, p b 0.05). As shown in Fig. 4, the average Epworth Sleepiness Scale scores of both the week prior to starting the study and the sleep deprived week, were compared to that of the sleep extension week. A repeated measures analysis of variance confirmed that the Epworth Sleepiness Scale scores also decreased significantly from the first week to the second week (F(2,22) = 19.10, p b 0.01). See Table 1 for a comparison of the means. Fig. 3. Change in serving accuracy of each participant at week 2 (post-sleep extension period).
2.7. Statistical analysis Multiple t tests and a repeated measures analysis of variance were performed using the statistical package SPSS. Since there were no significant gender differences in either serving accuracy, subjective sleepiness measures, or amount of sleep obtained, data for males and females were combined and tested together in all analyses. 3. Results 3.1. Sleep extension improves serving accuracy This study compared performance on a complex sensorimotor task during periods of predicted sleep deprivation and periods of predicted sleep extension. A correlated groups t test was used to compare the mean number of accurate serves at the end of the sleep deprived week with the mean number of accurate serves at the end of the sleep extension week (t(11) = −1.97, p b 0.05, one-tailed). The accuracy of the athletes' tennis serves improved significantly after sleep extension for 7 days (Fig. 2). Fig. 3 shows the percent change (improvement or decline) in the serving accuracy from the sleep deprived week to the sleep extension week of each individual player. 3.2. Confirmation of sleep status In order to confirm that the tennis players were in a sleep deprived state during week 1 of the study, hours of sleep obtained every night/ day, including naps, were reported for the duration of that week. The players were then asked to extend their sleep by obtaining at least 9 h every night/day during the second week, in an attempt to decrease the players' sleep debt. Table 1 shows the mean number of hours of sleep obtained in the first week and in the second week during sleep extension. A paired samples t test indicated that the players did indeed
3.4. Caffeine intake The amount of caffeine consumed during the sleep deprived week was compared with that of the sleep extension week (Table 1). A correlated groups t test revealed no significant difference between the amount of caffeine consumed during the first week and the amount consumed during the second week (t(11) = −0.20, p = 0.41), thereby eliminating caffeine as a potential confounding variable. 4. Discussion Results of this study demonstrate that increases (slightly less than 2 h for seven days/nights) in sleep duration yield significant benefits in accuracy and performance of varsity tennis players' serves. Participants obtained significantly more sleep during the sleep extension period (week 2) than during the sleep deprived period (week 1; habitual sleep). This regimen of sleep extension also had broad positive affects on self-reported wakefulness, as measured by both the Stanford Sleepiness Scale and the Epworth Sleepiness Scale. The fact that there were no significant gender differences in either serving accuracy or wakefulness measures could further reflect the central importance of sleep in preparing the body for optimum performance. In addition, there was no significant difference between the amount of caffeine consumed during the sleep deprived week and the sleep extension week. The tennis serve is an ideal model of a complex, integrative, sensorimotor skill that requires concentration, coordination, perceptual memory, motivation, balance, strength, and the activation and synchronization of multiple neural systems. Therefore, sleep extension could have positively affected the accuracy of the serve by enhancing a number of processes that are necessary to achieve an optimal serve. Nine out of the twelve participants increased their serving accuracy after the period of sleep extension, while the serving accuracy of three of the players decreased. The significant increase in hours of sleep correlated with a significant improvement in the accuracy of the tennis players' serves, however it is important to address alternative explanations. It is improbable
Table 1 Characteristics of participants' tennis serving accuracy, sleep, and caffeine consumption before and after sleep extension.
Accurate serves out of 50 (deuce & add side together), mean (SD) Accurate serves on the deuce side (out of 25), mean (SD) Accurate serves on the add side (out of 25), mean (SD) Sleep per night (hours), mean (SD) Stanford Sleepiness Scale score, mean (SD) Epworth Sleepiness Scale score, mean (SD) Caffeinated beverages consumed, mean (SD)
Week 1 (before sleep extension)
Week 2 (after sleep extension)
p value
17.83 (7.84) 9.25 (5.28) 8.58 (3.32) 7.14 (0.87) 3.56 (0.77) 12.15 (4.20) 0.89 (0.47)
20.92 (6.80) 10.67 (3.99) 10.25 (3.49) 8.85 (0.60) 2.67 (0.64) 5.67 (2.46) 0.94 (0.77)
b0.05
b0.05 b0.05 b0.01 0.41
544
J. Schwartz, R.D. Simon Jr / Physiology & Behavior 151 (2015) 541–544
in varsity tennis players' second serves. These results indicating that sleep enhances athletic performance add to previous research revealing that sleep is essential for the formation of motor and perceptual skill memories. Experimental studies with larger samples and objective sleep measures that aim to extend and/or improve sleep are warranted to examine whether performance is enhanced in college-aged students, and in turn, whether downstream harmful outcomes associated with sleep deprivation are reduced. This study also suggests that more research on the effects of sleep extension, rather than sleep restriction, could be informative in college age subjects. Sleep is necessary for optimal performance of motor skills, and it may be necessary before achieving the maximum improvement from practice. Sleep should be an integral part of athletic training programs, and coaches should emphasize the importance of sleep.
References Fig. 4. Epworth Sleepiness Scale scores at baseline, after week 1 (post-habitual sleep period), and after week 2 (post-sleep extension period).
that the increase in accuracy of the serves was due to learning or practice from the first serving session. The participants in this study were college varsity tennis players who practice their serves on a daily basis, so it is unlikely that one extra practice session during this study would improve such a routine task. If the participants had not increased sleep between the two serving sessions, it is unlikely that the accuracy of their serves would have increased appreciably [16]. In fact, previous results reveal that the brain structures that are activated during the learning or training of a serial motor task must be reactivated in order for these motor memories to be consolidated, and the reactivation of these brain structures is most apparent during REM sleep [16]. The Epworth Sleepiness Scale scores were very similar when administered one week prior to starting the study and one week into it, after the players had followed their normal sleeping habits for one week (sleep deprived week). However, their reported feelings of sleepiness then significantly decreased after the period of sleep extension. Therefore, these athletes did indeed feel better rested than they had in at least two weeks, which certainly could have contributed to the increase in accuracy of their serves. This is consistent with many previous studies. For example, Matsumoto and colleagues found that feelings of tiredness negatively correlated with accuracy and performance [20]. The ratings of the Stanford Sleepiness Scale further confirmed this decrease in feelings of sleepiness from the sleep deprived to the sleep extension week. 5. Limitations Several limitations should be mentioned. First, typical collegiate tennis teams include 16–24 male and female members, which limited the number of potential participants that could be enrolled in the study, thereby resulting in a small sample size of 12. Nonetheless, this study included a substantial percentage of the tennis team. Second, measures of sleep duration were based on self-report, which may not be representative of habitual sleep and could have resulted in misclassification bias [21]. Nonetheless, data indicate moderate correlations between selfreported sleep hours and actigraphy (Pearson's r = 0.53) [22]. 6. Conclusions Sleep extension significantly improved the accuracy of tennis serves in the majority of these varsity tennis players. The results of this study suggest that extending sleep in college aged people to the recommended 9 h has the potential to improve the accuracy of a complicated multifaceted task — serving in tennis. Specifically, the current findings demonstrate that sleep extension from a baseline of approximately 7 h per night to nearly 9 h per night resulted in significant improvement
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