Over the past 10 years, the sports medicine community has become increasingly aware of the relationship between sleep deprivation and diminished athletic performance. Sleep deprivation appears to be epidemic on campus and negatively influences every aspect of students’ lives. College students who sleep poorly have more health issues, higher stress and depression, and are far more likely to use over-the-counter stimulant medications to remain awake and use alcohol to promote sleep compared with good sleepers (1). Physiologic impacts of sleep deprivation include negative influences on hormone balance and metabolism (2), diminished muscle glycogen storage (3), and increased susceptibility to illness (4). Sleep deprivation is associated with depression and negative mood (5) and is detrimental to learning, memory, and judgment (6). Longer sleep is correlated with improved mood and academic performance (6). Despite this, sleep medicine receives relatively little coverage in medical school, primary care residency, or sports medicine fellowship curricula.
Chronic suboptimal sleep (that is, sleep deprivation) impairs neurologic function and as a result, negatively impacts performance, appropriate cognitive processing, and mood. The collegiate athletic population is subject to chronic sleep deprivation for multiple reasons. Mah et al. (7) recently characterized the self-reported sleep habits of sports teams at a Division 1 institution. Nearly 40% had fewer than the recommended 7 h sleep/night. While there is considerable variability in individual sleep requirements, the collegiate population experience acute and chronic sleep deprivation from the combined demands of collegiate academic and athletic schedules (7). This group is further at risk for circadian rhythm disturbances that are both inherent to the age group and acquired by habit through exposure to light-emitting electronics (7). This review examines recent evidence for the effect of sleep deprivation on the collegiate athlete’s performance, learning, judgment, and mood as well as the evidence for potential interventions for this at-risk population.
Basic Sleep Physiology and Metabolic Effects
Understanding the timing and metabolic functions of different sleep types is essential to understanding the mechanism of sleep deprivation's pervasive effects. Most individuals function well for 16 h of wake time, with 8 h of sleep and recovery (5,8). Sleep and alertness is related to circadian rhythm; sleep onset naturally coincides with decreased light (sunset), and waking typically coincides with sunrise (8). During waking hours, the circadian rhythm effect waxes and wanes. During wake, this leads to periods where an individual is more alert and during other periods is less alert. Simply put, there are biological connections to the environment that promote/discourage sleep. Individual performance varies with time of day relative to the most-activating periods of the individual's circadian rhythm (9). Usually, this becomes relevant for travelling athletes crossing multiple time zones. While “jet lag” influences performance, the athlete's biological clock can be artificially moved without travel by inappropriate light cues from electronic devices during dark hours (10). Although important, circadian rhythm impact on the athletic performance is beyond this discussion and has recently been systematically reviewed (11).
“Sleep quality” can be a subjective or objective description. More than the impression of “how well they slept,” “quality” should be viewed by the unique metabolic and physiologic roles that occur within each sleep stage (see Fig. 1). Human sleep has four distinct stages. Stage I is very light sleep. In this stage, representing about 5% of total sleep time, the sleeper often imagines falling and “jerks” awake — a “hypnic jerk.” The role of stage I is not fully understood. Stage II, also light sleep, is characterized by intermittent rapid bursts of coordinated neurologic activity called “sleep spindles.” During stage II, short-term memories stored in the hippocampus are moved to long-term storage in the cerebral cortex (12). Stage II may have a role in gross motor learning (13). Stage III is deep restorative (dreamless) sleep. Clearance of neurologic metabolic debris occurs (14) along with long-term memory consolidation; for example, a newly learned word list is converted to long-term memory during stage III (13). Stage REM sleep (dream sleep) further hones long-term memory.
Sleep stage organization during sleep is termed “sleep architecture” (Fig. 1). Stage III and stage REM cycle throughout the night in 90-min intervals (a sleep cycle). In early evening, stage III dominates, but with each subsequent cycle, REM increases. With adequate sleep, most individuals will cycle four to six times per night, averaging 50% stage III/stage REM and 50% of lighter stages (8). In chronic sleep restriction, stage I and stage III sleep are preserved, but stage II and stage REM are proportionally decreased in response to restricted sleep time (15). Not all sleep deprivation is identical; for example, waking 2 h early decreases total sleep time by 2 h (25% of recommended), but results in 50% of REM sleep loss.
Sleep Hygiene and the College Athlete
College dormitory life is inconsistent with good sleep habits, described as “sleep hygiene” (Table). Approximately 77% of college students have poor sleep hygiene (16,17). Total sleep amounts also are restricted: 25% of students reported <6.5 h per night and 70% reported <8 h per night (1). Sleep deprivation in the college setting is multifactorial. Perceived stress is a major contributor to poor sleep (1). Insomnia due to preperformance anxiety, sleep fragmentation (student waking during normal sleep), phone/tablet use, sleep curtailment due to academic demands, or early morning training contribute to chronic sleep shortening. Chronic sleep deprivation leads to significant sleep “debt” among collegiate athletes. To illustrate, Figure 2 shows a semester overlay of sleep and wake times for the shortest (5.2 h and longest (7.2 h) of the 14 tested athletes at a division 3 school (18). Sleep and wake times are relatively uniform in the early semester, consistent with other observations (1,19). In the second half of the semester, sleep habits deteriorate. Weekend bed times and rise times are generally later, consistent with previous studies of self-reported sleep habits of college students (1). Sleep restriction patterns change in response to academic and athletic schedules; students sleep less immediately before academic events, such as mid-terms or finals, and the night following a game or team event. Finally, while the recommended nightly sleep for this age to prevent health/performance decrements is 7 to 8 h per night (8,20), our athletes had 5.2 to 7.2 h per night. Total sleep deprivation for the short sleeper relative to the recommended 7 h per night over a semester was 200 h, representing almost 1600 h over a 4-year degree (66 d). The long-term health implications of this degree of sleep deprivation, if continued into the postcollege years, are significant. This small sample size only hints at the severity of the epidemic of sleep deficit on college campuses.
Sleep Deficit on Vigilance
Sleep interruption in the collegiate dormitory setting is intuitively common. Most studies focus on either the effect of 1 to 3 d of acute sleep deprivation or of longitudinally restricted sleep. The effects of sleep deprivation on perception are remarkable. When reaction time and vigilance were measured during acute sleep deprivation over 88 h, van Dongen and Dinges found that during the first 16 h of wake time, there were few mistakes. Thereafter, reaction time and missed cues increased in a pattern that followed the circadian rhythm and accumulated with progressive sleep loss (5). Moreover, one to two nights of recovery sleep did not completely restore performance to baseline, suggesting that a weekend of “catch up” sleep may not be adequate. Similar results were found for chronic sleep restriction. Examining the learning and vigilance of a population that included college-aged individuals, they examined 4, 6, and 8 h sleep/night over a 2-wk period (15). There was a dose-dependent deterioration over the 2 wk where those sleeping 8 h per night had preserved performance, but lapses increased for the 6 h per night group and even more for the 4 h sleep per night group. Again, 2 d of recovery sleep was insufficient for the 4 h and 6 h sleep per night groups to return to the 8 h per night group baseline. Of significance, sleep-deprived participants consistently underestimated their degree of impairment (5,21). Chronic sleep deprivation over 2 wk was just as performance-limiting as 2 d of total sleep deprivation. Sport requires recognition, correct analysis, and then reaction/execution based on vigilance; sleep deprivation increases errors in judgment and results in slower reaction time.
1.4.Sleep and Mood
Sleep time and emotional health are connected (22). Inability to fall asleep or early wakening is one of the screening criteria for clinical depression. Sleep deprivation as an independent risk factor for mood disorders is less well established. After one night of sleep deprivation, physician's moods demonstrated statistically significant increased tension, anger, fatigue, confusion, irritability, feeling jittery, and sleepiness, with a decrease in vigor, energy, and confidence (23). Finan et al. (24) tested a group that included college-aged students after having either 8 h of uninterrupted sleep or after 8 h of sleep with every-hour forced awakenings. A single night of interrupted sleep diminished the ability to recognize and respond to positive emotional stimuli; response to negative stimuli was preserved. A second study found that two consecutive days of interrupted sleep diminished stage III and lead to significantly lower positive mood scores (25). Chronic sleep deprivation alters facial cues which are recognized by others and may influence social judgments (26). Chronic sleep deprivation could put student athletes at risk for a “negative spiral”; sleep deficiency-fueled increase in moodiness and irritability; fatigued/angry facial cues lead to suboptimal interactions with staff, coaches, and peers, leading to negative social outcomes and isolation, perpetuating the cycle.
Sleep and Concussion Testing
Sleep deprivation may influence data obtained during presport and in-sport testing (27). Baseline computer-based neurocognitive measurements are frequently performed for concussion prior to the start of competition. Interrupted sleep is a hallmark of the immediate postconcussion period (28,29), and history of concussion may influence sleep and quality-of-life indices in the collegiate population (30). In their study, low sleep quantity the night before testing was correlated with greater number of symptoms and higher symptom severity score (27). Examining the effect of prior night's sleep on IMPACT® scores, McClure et al. found that athletes with fewer than 7 h of sleep before baseline testing performed worse on reaction time, verbal memory, and visual memory scores compared with longer sleeping controls (31). Finally, in the postconcussion period, athletes who were concussed and had lost more than 1 h of their usual sleep per night report more symptoms and increased severity on serial evaluations (28). Evaluating sleep may be indicated before baseline testing is performed since return to play decisions are in part made in comparison to baseline.
Appropriate intervention in the college athlete depends on the underlying cause of sleep deprivation. Sleep itself will not improve performance; the preponderance of data suggests that neurologic function is improved by sleep when the individual is sleep-deprived. Mah et al. well illustrated the effect of sleep prolongation (32) on athletic performance. A group of Stanford basketball players were evaluated on skill, speed, and mood. After a baseline of 4 wk of their usual sleep (approximately 6.7 h per night by actigraphy), athletes were asked to increase sleep over the next 5 to 7 wk (to an average of 8.5 h per night by actigraphy). After sleep extension, the group demonstrated significant improvements in free throw shooting (9%), three-point shooting (9.2%), and faster sprint times after a 6-wk trial of sleep extension. In addition, they had improvements in fatigue and sleepiness scales as well as mood profile scores.
The effect of increased sleep on performance has been replicated in other sports. An acute night of sleep deprivation (5 h vs usual 7.5 h) resulted in decreased tennis serve accuracy in college-aged semiprofessional players (33). In a similar population of college tennis players, 1 wk of 2-h sleep extension (to 9 h daily) improved serve accuracy from 35% to 42% and improved sprint times (34). Sleep deprivation did not influence power or performance in collegiate weight lifters, but did adversely affect mood, confusion, fatigue, and vigor (35). Most studies have small study populations and low power but suggest that suboptimal sleep time impairs neurologic function; whereas improvements in sleep time allow for improved neurologic function and therefore improved performance in terms of accurate replication of learned gross motor-firing patterns. Sleep extension in this context improves performance.
In search of the competitive edge, educating the college athlete about sleep and its connection to performance is the new battleground. The traditional approach to addressing college student sleep issues has been didactic in nature. These include classroom interventions and either direct (36) or online-based education (37,38). Specific interventions, such as cognitive behavioral therapy, can be very helpful but is usually reserved for students with specific sleep issues, such as insomnia, due to difficulty falling asleep because of pervasive thoughts or early waking from anxiety or depression (39,40).
Educating students about basic sleep hygiene and sleep-habit optimization can alter their behavior. Unfortunately, many traditional “sleep hygiene” tenets (Table) fall outside of the control of a dormitory-dwelling athlete. For those under their control, a more tailored, personalized approach may have greater success. Herschel and O’Brien (41) recently reported a relatively individualized online educational tool in a randomized controlled study targeting sleep hygiene in college students. Students were referred to a web-based tool (sleeptostayawake.org), and a personalized profile was developed with education, including the importance of total sleep time, regular bed and wake times, avoiding electronics in the hours leading up to bed, and obtaining adequate sleep prior to examinations by limiting “all-nighters.” Only about 60% of those randomized for intervention participated in the study. Of those who did, about 75% reported incorporating what they learned into their sleep habits. At 8 wk, students who participated in the online education had significantly lower depression scores, improved sleep quality, and reported improvements in self-reported sleep-promoting behaviors, including stopping electronics earlier, earlier wake times during the week, having a more regular sleep schedule, and taking fewer naps. Their findings are particularly important in that the improvements were seen at 8 wk, suggesting that early-semester intervention may allow for behavior change prior to the deterioration in sleep patterns typically seen in the second half of the semester (Fig. 2).
Obtaining a simple sleep history during the preparticipation physical or during introductory student-athlete meetings that precede most athletic seasons may be helpful. Those athletes who score poorly on Epworth Sleepiness Scale (42), Pittsburgh Sleep Quality Index (43), or newer tools designed by sports physicians for elite athletes (44) may indicate need for intervention.
While education is effective, it does require time for the athlete to adjust and implement change. For faster results, students will often turn to either over-the-counter or prescribed medications to increase alertness or promote sleep. Caffeine is among the most commonly used and will promote alertness, especially the day after shortened sleep. Caffeine blocks adenosine receptors; adenosine accumulates in the CSF during wakefulness and promotes sleep onset. Taken late in the day, caffeine can cause insomnia by preventing the onset of sleep.
There are several common medications for inducing sleep. All have side effects, and some can be addictive and should be used with caution in the collegiate population. Over-the-counter (diphenhydramine) or prescription (doxepin (3 to 6 mg) antihistamines will induce sleep but may have residual morning grogginess and so should be used with caution for sleep induction if early morning function is required. Trazedone, commonly prescribed for insomnia, is not recommended for the college population. Common medications classes, such as benzo-diazepines (temazepam) and nonbenzodiazepine benzodiazepine-receptor antagonists (Zolpidem), have variable half-lives and are effective for sleep induction. Both types of drugs are known to promote sleep while also suppressing stage III and stage REM sleep. This latter effect also is observed as a side effect in several commonly prescribed drugs, including the antidepressant selective-seritonin reuptake inhibitors (fluoxetine). Medications to promote sleep may be best used sparingly, for short durations, and in conjunction with an education program designed to retrain students’ sleep hygiene habits (45).
Napping is common among college athletes. At Stanford, 80% of athletes reported taking at least one nap during the week and 11% routinely took pregame/competition naps (7). Napping is associated with higher academic performance (6). Naps may be insufficient to overcome accumulated deficits from several months or years of poor sleep and the data on effectiveness is mixed; naps have been shown to mitigate the effect of short-term sleep deprivation (5). The rationale for naps being ergogenic is based on the observation that alertness, reaction time, and sprint performance improved with a 30-min afternoon nap in the setting of sleep restriction the previous night (46). In karate, a 30-min nap improved subjective feelings of alertness and fatigue regardless if there was adequate sleep or partial sleep deprivation the previous night. Naps improved both cognitive and physical performance (47). Other data are conflicting, with a 40-min afternoon nap contributing to some deterioration in cognitive task performance in college students who habitually nap (48). Petit et al. found that a 20-min nap did not improve cycling performance and also did not alter postexercise sleep architecture (49). Short naps of 30 min or less are likely restorative, particularly in the setting of sleep deprivation. They are most effective for performance preservation when stage II sleep is present (50), are less likely to have postnap grogginess (sleep inertia) than naps of longer duration which have potential to disturb normal sleep architecture.
Sleep is essential for health, mood, academic, and athletic performance. Deficient sleep, regardless of cause, decreases neurologic function and is manifested by decreased performance. Sleep extension or napping, particularly in the context of sleep deprivation, improves neurologic function and as a result improves performance. Although further research investigation is warranted, there is sufficient evidence to justify clinical inquiry regarding sleep and sleep habits of student athletes. Educational intervention provides an opportunity for athletes to make lifestyle changes with goals of better performance and improvement in health and mental well-being.
The author declares no conflict of interest and does not have any financial disclosures.
1. Lund H, Reider B, Whiting A, Prichard J. Sleep patterns and predictors of disturbed sleep in a large population of college students. J. Adolesc. Health
. 2010; 46:124–32.
2. Nedeltcheva AV, Kilkus JM, Imperial J, et al. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann. Intern. Med
. 2010; 153:435–41.
3. Skein M, Duffield R, Edge J, et al. Intermittent-sprint performance and muscle glycogen after 30 h of sleep deprivation. Med. Sci. Sports Exerc
. 2011; 43:1301–11.
4. Cohen S, Doyle WJ, Alper CM, et al. Sleep habits and susceptibility to the common cold. Arch. Intern. Med
. 2009; 169:62–7.
5. Van Dongen HP, Dinges DF. Sleep, circadian rhythms, and psychomotor vigilance. Clin. Sports Med
. 2005; 24:237–49, vii–viii.
6. Eliasson AH, Lettieri CJ. Early to bed, early to rise! Sleep habits and academic performance in college students. Sleep Breath
. 2010; 14:71–5.
7. Mah CD, Kezirian EJ, Marcello BM, Dement WC. Poor sleep quality and insufficient sleep of a collegiate student-athlete population. Sleep Health
. 2018; 4:251–7.
8. Lee-Chiong T. Sleep: A Comprehensive Handbook
. Hoboken (NJ): John Wiley & Sons; 2006.
9. Anderson A, Murray G, Herlihy M, et al. Circadian effects on performance and effort in collegiate swimmers. J. Circadian Rhythms
. 2018; 16:8, pp. 1–9.
10. Orzech KM, Grandner MA, Roane BM, Carskadon MA. Digital media use in the 2 h before bedtime is associated with sleep variables in university students. Comput. Hum. Behav
. 2016; 55:43–50.
11. Thun E, Bjorvatn B, Flo E, et al. Sleep, circadian rhythms, and athletic performance. Sleep Med. Rev
. 2015; 23:1–9.
12. Mizuseki K, Miyawaki H. Hippocampal information processing across sleep/wake cycles. Neurosci. Res
. 2017; 118:30–47.
13. Christova M, Aftenberger H, Nardone R, Gallasch E. Adult gross motor learning and sleep: is there a mutual benefit? Neural Plast
. 2018; 2018:3076986. Epub 2018/08/13. doi: 10.1155/2018/3076986. PubMed PMID: 30186317; PubMed Central PMCID: PMCPMC6110005.
14. Xie L, Kang H, Xu Q, et al. Sleep drives metabolite clearance from the adult brain. Science
. 2013; 342:373–7.
15. Van Dongen H, Maislin G, Mullington J, Dinges D. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep
. 2003; 26:117–26.
16. Kaur G, Singh A. Sleep hygiene, sleep quality and excessive daytime sleepiness among Indian college students. J. Sleep Med. Disorder
. 2017; 4:1076–83.
17. Gellis LA, Park A, Stotsky MT, Taylor DJ. Associations between sleep hygiene and insomnia severity in college students: cross-sectional and prospective analyses. Behav. Ther
. 2014; 45:806–16.
18. Bolin D, Buriak J, Creasy J, et al. Longitudinal correlation of sleep time, strength gains, and performance in collegiate baseball players: a pilot study. American College of Sports Medicine Annual Meeting; Minneapolis, MN. Med. Sci. Sports Exerc
. 2018; 50:800.
19. Hershner SD, Chervin RD. Causes and consequences of sleepiness among college students. Nat. Sci. Sleep
. 2014; 6:73–84.
20. Watson NF, Badr MS, Belenky G, et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep
. 2015; 38:843–4.
21. Markwald R, Bessman S, Drummond S, et al. Performance during unplanned night time awakenings and following disrupted sleep. J. Sci. Med. Sport
. 2017; 20:S18.
22. Baglioni C, Spiegelhalder K, Lombardo C, Riemann D. Sleep and emotions: a focus on insomnia. Sleep Med. Rev
. 2010; 14:227–38.
23. Saadat H, Bissonnette B, Tumin D, et al. Time to talk about work-hour impact on anesthesiologists: the effects of sleep deprivation on Profile of Mood States and cognitive tasks. Paediatr. Anaesth
. 2016; 26:66–71.
24. Finan PH, Quartana PJ, Remeniuk B, et al. Partial sleep deprivation attenuates the positive affective system: effects across multiple measurement modalities. Sleep
. 2017; 40:zsw017.
25. Finan PH, Quartana PJ, Smith MT. The effects of sleep continuity disruption on positive mood and sleep architecture in healthy adults. Sleep
. 2015; 38:1735–42.
26. Sundelin T, Lekander M, Kecklund G, et al. Cues of fatigue: effects of sleep deprivation on facial appearance. Sleep
. 2013; 36:1355–60.
27. Mihalik JP, Lengas E, Register-Mihalik JK, et al. The effects of sleep quality and sleep quantity on concussion baseline assessment. Clin. J. Sport Med
. 2013; 23:343–8.
28. Hoffman NL, Weber ML, Broglio SP, et al. Influence of postconcussion sleep duration on concussion recovery in collegiate athletes. Clin. J. Sport Med
. 2017. Epub 2017/11/16. doi: 10.1097/JSM.0000000000000538. PubMed PMID: 29189340.
29. Hoffman NL, O'Connor PJ, Schmidt MD, et al. Differences in sleep between concussed and nonconcussed college students: a matched case-control study. Sleep
. 2019; 42. doi: 10.1093/sleep/zsy222. PubMed PMID: 30452734.
30. Blake AL, McVicar CL, Retino M, et al. Concussion history influences sleep disturbances, symptoms, and quality of life in collegiate student-athletes. Sleep Health
. 2019; 5:72–7.
31. McClure DJ, Zuckerman SL, Kutscher SJ, et al. Baseline neurocognitive testing in sports-related concussions: the importance of a prior night's sleep. Am. J. Sports Med
. 2014; 42:472–8.
32. Mah CD, Mah KE, Kezirian EJ, Dement WC. The effects of sleep extension on the athletic performance of collegiate basketball players. Sleep
. 2011; 34:943–50.
33. Reyner LA, Horne JA. Sleep restriction and serving accuracy in performance tennis players, and effects of caffeine. Physiol. Behav
. 2013; 120:93–6.
34. Schwartz J, Simon RD. Sleep extension improves serving accuracy: a study with college varsity tennis players. Physiol. Behav
. 2015; 151:541–4.
35. Blumert PA, Crum AJ, Ernsting M, et al. The acute effects of twenty-four hours of sleep loss on the performance of national-caliber male collegiate weightlifters. J. Strength Cond. Res
. 2007; 21:1146–54.
36. Tetsuo H, Wada K, Tsuji F, et al. Intervention study using a leaflet entitled “Three benefits of ‘Go to bed early! Get up early! and intake nutritionally rich breakfast!’ a message for athletes” to improve the soccer performance of university soccer team. Sleep Biol. Rhythms
. 2016; 14:S65–74.
37. Quan S, Anderson J, Hodge G. Use of a supplementary internet based education program improves sleep literacy in college psychology students. J. Clin. Sleep Med
. 2013; 9:155–60.
38. Kloss JD, Nash CO, Walsh CM, et al. A “sleep 101” program for college students improves sleep hygiene knowledge and reduces maladaptive beliefs about sleep. Behav. Med
. 2016; 42:48–56.
39. Trockel M, Manber R, Chang V, et al. An e-mail delivered CBT for sleep-health program for college students: effects on sleep quality and depression symptoms. J. Clin. Sleep Med
. 2011; 7:276–8.
40. Morris J, Firkins A, Millings A, et al. Internet-delivered cognitive behavior therapy for anxiety and insomnia in a higher education context. Anxiety Stress Coping
. 2016; 29:415–31.
41. Hershner S, O'Brien LM. The impact of a randomized sleep education intervention for college students. J. Clin. Sleep Med
. 2018; 14:337–47.
42. Johns M. A new method for measuring daytime sleepiness: the Epworth sleepiness scale. Sleep
. 1991; 14:540–5.
43. Buysse DJ, Reynolds CF, Monk TH, et al. The Pittsburgh Sleep Quality Index: a new instrument for psychiatric practice and research. Psychiatry Res
. 1989; 28:193–213.
44. Samuels C, James L, Lawson D, Meeuwisse W. The Athlete Sleep Screening Questionnaire: a new tool for assessing and managing sleep in elite athletes. Br. J. Sports Med
. 2016; 50:418–22.
45. Dujardin S, Pijpers A, Pevernagie D. Prescription drugs used in insomnia. Sleep Med. Clin
. 2018; 13:169–82.
46. Waterhouse J, Atkinson G, Edwards B, Reilly T. The role of a short post-lunch nap in improving cognitive, motor, and sprint performance in participants with partial sleep deprivation. J. Sports Sci
. 2007; 25:1557–66.
47. Daaloul H, Souissi N, Davenne D. Effects of napping on alertness, cognitive, and physical outcomes of karate athletes. Med. Sci. Sports Exerc
. 2019; 51:338–45.
48. Ru T, Chen Q, You J, Zhou G. Effects of a short midday nap on habitual nappers' alertness, mood and mental performance across cognitive domains. J. Sleep Res
. 2019; 28:e12638.
49. Petit E, Mougin F, Bourdin H, et al. A 20-min nap in athletes changes subsequent sleep architecture but does not alter physical performances after normal sleep or 5-h phase-advance conditions. Eur. J. Appl. Physiol
. 2014; 114:305–15.
50. Hayashi M, Motoyoshi N, Hori T. Recuperative power of a short daytime nap with or without stage 2 sleep. Sleep
. 2005; 28:829–36.