The Most Neglected Training Variable
When athletes talk about what holds back their progress, the focus is usually on training, nutrition, or recovery tools. Rarely do they mention sleep. Yet the literature is consistent: reductions in sleep impair nearly every marker of performance and recovery.
In one landmark trial, researchers restricted participants to six hours of sleep per night for two weeks. By the end, their reaction times were comparable to someone who had been awake for 24 straight hours, even though participants reported feeling only mildly fatigued (1). This illustrates a key point: we are poor judges of how much sleep loss affects us. The body does not adapt to short sleep, performance simply declines.
For athletes, this has major implications. Sleep is not passive downtime. It is the hidden training block where the body repairs tissue, restores glycogen, regulates hormones, consolidates skills, and resets the nervous system. When sleep is consistently cut short, progress is compromised regardless of the effort put into training or nutrition.
What Your Body Builds While You’re Asleep
Sleep is not a uniform state. Each night cycles between light sleep, deep slow-wave sleep (SWS), and rapid eye movement (REM) sleep, with each stage contributing differently to recovery and adaptation.
Hormonal regulation during SWS. The first half of the night is dominated by SWS, when growth hormone is released in large pulses. Growth hormone supports tissue repair, muscle remodeling, and recovery from training2. Testosterone also follows a sleep-dependent rhythm, with levels rising overnight and peaking in the morning. When sleep is restricted, testosterone declines and cortisol increases, creating a hormonal environment less favorable for muscle repair and hypertrophy (3).
Energy restoration and metabolism. Deep sleep also plays a role in glycogen replenishment. Athletes who restrict sleep show reduced glycogen repletion, higher perceived exertion, and earlier fatigue in endurance tests (4). Even short-term sleep restriction impairs glucose tolerance and insulin sensitivity, altering fuel utilization (5).
Skill and motor learning. Later in the night, REM sleep is especially important for consolidating new motor patterns. Research has shown that sleep enhances the retention of complex motor tasks and improves accuracy and efficiency in movement (6). In practical terms, technical changes in lifting, running form, or pacing strategies are more likely to “stick” when adequate REM sleep follows practice.
Autonomic recovery. During SWS, parasympathetic activity rises while sympathetic activity decreases, reflected in improved heart rate variability (HRV) and lower resting heart rate (7). Chronic sleep restriction produces the opposite pattern, with reduced HRV and heightened sympathetic tone, making training stress feel more demanding (8).
Immune function. Sleep is also a key regulator of immune readiness. Athletes who consistently sleep fewer than seven hours are more likely to develop upper respiratory tract infections compared to those who sleep longer (9).
Sleep is not just recovery in the background. It directly influences strength, endurance, skill acquisition, injury risk, and cognitive performance.
Strength and muscle growth. In controlled experiments, partial sleep restriction lowered testosterone and increased cortisol (3). These hormonal shifts create a less favorable environment for maintaining or building lean mass. More recently, sleep loss has been shown to blunt muscle protein synthesis, the very process that repairs and builds new tissue after training (10). Even with optimal nutrition, restricted sleep can slow hypertrophy and compromise strength gains.
Endurance and conditioning. Glycogen availability is central for endurance performance, and sleep restriction interferes with its restoration. Athletes deprived of adequate sleep show shorter time-to-exhaustion, higher perceived exertion, and slower metabolic recovery following exercise (4,11). From a performance standpoint, insufficient sleep makes submaximal work feel harder and maximal efforts end sooner.
Skill and motor learning. Motor adaptation does not end with practice. Sleep plays an essential role in consolidating learned skills. Studies have demonstrated that both NREM spindles and REM sleep improve retention of complex motor sequences and accuracy of movement execution6. For athletes refining lifting technique, stride mechanics, or pacing strategies, sleep is part of the practice itself.
Injury risk and recovery. Epidemiological data show that athletes sleeping less than eight hours per night are at significantly greater risk of injury. In adolescent athletes, reduced sleep was associated with a 1.7-fold higher risk of sustaining an injury compared with peers who slept longer 12). Insufficient sleep increases inflammation, slows reaction time, and reduces coordination, all of which compound injury risk under training stress.
Cognitive performance and mental readiness. Reaction time, decision-making, and perceived exertion are all impaired by sleep loss. In the Stanford basketball study, when players extended their nightly sleep toward nine hours, sprint times improved and shooting accuracy increased (13). The intervention was simple — more time in bed — yet the outcomes were measurable improvements in performance.
Together, these findings demonstrate that sleep is not peripheral to performance. It is a determining factor across strength, endurance, skill, and resilience.
Building Better Sleep Habits
The most effective strategies for improving sleep are straightforward. They do not require expensive tools, but they do require consistency.
Environment matters.
- A cool, dark, and quiet room promotes deeper sleep. Experimental studies suggest that ambient temperatures around 65–68°F (18–20°C) are optimal for most individuals (14).
- Blackout curtains and minimizing noise further support both sleep onset and continuity.
Consistency is key.
- Going to bed and waking up at similar times strengthens circadian rhythms.
- Stable rhythms improve hormone release patterns, core body temperature regulation, and daily energy levels (15).
Pre-sleep routine.
- Screen use is often blamed for poor sleep, but it may be less about the blue light itself and more about delayed bedtime and cognitive stimulation.
- Studies show that reducing screen exposure or shifting to lower-stimulation activities (reading, journaling, stretching) before bed supports faster sleep onset and longer total sleep duration (16) .
- For many athletes, simply setting a consistent cutoff time for electronics is more impactful than any single device or filter.
Duration and sleep extension.
- Athletes benefit from the higher end of the general recommendation of 7–9 hours. In practice, 8–9 hours often produces measurable improvements in performance.
- In Mah et al. (13), basketball players who extended their sleep toward nine hours per night improved sprint performance and shooting accuracy.
Banking sleep.
- Increasing sleep duration before anticipated restriction helps buffer against subsequent declines in performance. Studies show that extending sleep in the days before competition reduces the impact of short sleep on reaction time and endurance capacity (1).
Naps as a supplement.
- Short naps of 20–30 minutes restore alertness and cognitive performance.
- Longer naps of about 90 minutes provide a full sleep cycle, supporting both recovery and motor learning (18).
- Naps are valuable but should complement, not replace, sufficient nighttime sleep.
These strategies represent the “big rocks” for athletes. Rather than chasing smaller hacks, prioritizing environment, consistency, duration, and simple routines delivers the largest return on investment.
Sleep as a Training Variable
When you step back, the evidence is clear. Sleep is not a luxury. It is a cornerstone of performance and recovery. It regulates hormones, supports protein synthesis, restores glycogen, strengthens motor learning, reduces injury risk, and steadies cognitive function. Every adaptation an athlete seeks — stronger lifts, better conditioning, sharper skills — is reinforced or blunted by what happens during sleep.
It helps to think of recovery in terms of a hierarchy:
- Foundation: Sleep
- Next layer: Nutrition and hydration
- Above that: Training structure and load management
- Final layer: Supplements and recovery tools
Supplements, gadgets, and advanced strategies all have their place, but none of them can compensate for chronically poor sleep. For athletes who want to perform at their highest level, sleep should be planned, protected, and treated with the same seriousness as training sessions or nutrition.
The real superpower is not in doing more, but in adapting better. Sleep is where that happens.
References
1. Van Dongen HPA, Maislin G, Mullington JM, Dinges DF. 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(2):117-126. doi:10.1093/sleep/26.2.117
2. Van Cauter E, Holmback U, Knutson K, et al. Impact of sleep and sleep loss on neuroendocrine and metabolic function. Horm Res. 2007;67 Suppl 1:2-9. doi:10.1159/000097543
3. Leproult R, Van Cauter E. Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA. 2011;305(21):2173-2174. doi:10.1001/jama.2011.710
4. Skein M, Duffield R, Edge J, Short MJ, Mündel T. Intermittent-sprint performance and muscle glycogen after 30 h of sleep deprivation. Med Sci Sports Exerc. 2011;43(7):1301-1311. doi:10.1249/MSS.0b013e31820abc5a
5. Spiegel K, Leproult R, Van Cauter E. Impact of sleep debt on metabolic and endocrine function. Lancet Lond Engl. 1999;354(9188):1435-1439. doi:10.1016/S0140-6736(99)01376-8
6. Walker MP, Brakefield T, Morgan A, Hobson JA, Stickgold R. Practice with sleep makes perfect: sleep-dependent motor skill learning. Neuron. 2002;35(1):205-211. doi:10.1016/s0896-6273(02)00746-8
7. Trinder J, Kleiman J, Carrington M, et al. Autonomic activity during human sleep as a function of time and sleep stage. J Sleep Res. 2001;10(4):253-264. doi:10.1046/j.1365-2869.2001.00263.x
8. Sauvet F, Leftheriotis G, Gomez-Merino D, et al. Effect of acute sleep deprivation on vascular function in healthy subjects. J Appl Physiol Bethesda Md 1985. 2010;108(1):68-75. doi:10.1152/japplphysiol.00851.2009
9. Prather AA, Janicki-Deverts D, Hall MH, Cohen S. Behaviorally Assessed Sleep and Susceptibility to the Common Cold. Sleep. 2015;38(9):1353-1359. doi:10.5665/sleep.4968
10. Saner NJ, Lee MJC, Pitchford NW, et al. The effect of sleep restriction, with or without high-intensity interval exercise, on myofibrillar protein synthesis in healthy young men. J Physiol. 2020;598(8):1523-1536. doi:10.1113/JP278828
11. Oliver SJ, Costa RJS, Laing SJ, Bilzon JLJ, Walsh NP. One night of sleep deprivation decreases treadmill endurance performance. Eur J Appl Physiol. 2009;107(2):155-161. doi:10.1007/s00421-009-1103-9
12. Milewski MD, Skaggs DL, Bishop GA, et al. Chronic lack of sleep is associated with increased sports injuries in adolescent athletes. J Pediatr Orthop. 2014;34(2):129-133. doi:10.1097/BPO.0000000000000151
13. Mah CD, Mah KE, Kezirian EJ, Dement WC. The effects of sleep extension on the athletic performance of collegiate basketball players. Sleep. 2011;34(7):943-950. doi:10.5665/SLEEP.1132
14. Okamoto-Mizuno K, Mizuno K. Effects of thermal environment on sleep and circadian rhythm. J Physiol Anthropol. 2012;31(1):14. doi:10.1186/1880-6805-31-14
15. Czeisler CA, Gooley JJ. Sleep and circadian rhythms in humans. Cold Spring Harb Symp Quant Biol. 2007;72:579-597. doi:10.1101/sqb.2007.72.064
16. Exelmans L, Van den Bulck J. Bedtime mobile phone use and sleep in adults. Soc Sci Med 1982. 2016;148:93-101. doi:10.1016/j.socscimed.2015.11.037
17. Rupp TL, Wesensten NJ, Bliese PD, Balkin TJ. Banking sleep: realization of benefits during subsequent sleep restriction and recovery. Sleep. 2009;32(3):311-321. doi:10.1093/sleep/32.3.311
18. Lastella M, Memon AR, Vincent GE. Global Research Output on Sleep Research in Athletes from 1966 to 2019: A Bibliometric Analysis. Clocks Sleep. 2020;2(2):99-119. doi:10.3390/clockssleep2020010
