A limited number of studies have examined the effects of sleep deprivation on athletic performance. Souissi et al.  measured cycling maximal power, peak power and mean power pre and post 24 and 36 h of sleep deprivation. Up to 24 h of waking, anaerobic power variables were not affected, but were impaired after 36 h without sleep . Bulbulian et al.  examined knee extension and flexion peak torque before and after 30 h of sleep deprivation in trained men. Isokinetic performance decreased significantly following sleep deprivation. In support of the contention that the effects of sleep deprivation are task specific, 64 h of sleep deprivation significantly reduced vertical jump performance and isokinetic knee extension strength, while isometric strength and 40 m sprint performance were unaffected .
Blumert et al.  examined the effects of 24 h of sleep deprivation in nine US college-level weightlifters in a randomized counterbalanced design. There were no differences in any of the performance tasks (snatch, clean and jerk, front squat, and total volume load and training intensity) following 24 h of sleep deprivation when compared with no sleep deprivation . However, mood state, as assessed by the profile of mood states, was significantly altered, with confusion, vigour, fatigue and total mood disturbance all negatively affected by sleep deprivation.
Reductions in endurance running performance have been observed following 24 h of sleep deprivation . Eleven men performed a 30-min preload treadmill run at 60 % maximal oxygen uptake followed by a 30-min self-paced distance test following a normal night’s sleep (496 ± 18 min) and a night without sleep (0 min). Following the night of sleep deprivation, less distance was covered during the distance test (6,037 ± 757 m) when compared with control (6,224 ± 818 m; mean ± SD). The authors suggested that in the absence of significant alterations in physiological parameters and pacing, the reduced performance was likely to be a result of an increased perception of effort.
Skein et al.  reported significant decreases in mean and total sprint time following 30 h of sleep deprivation in ten male team-sport athletes. In addition, sleep deprivation resulted in altered repeat sprint pacing strategies, reduced muscle glycogen content, reduced peak voluntary force, reduced voluntary activation, and increased perceptual strain . The authors suggested that altered afferent feedback from reduced muscle glycogen content and/or increased perceptual strain may have reduced muscle recruitment and thereby reduced repeated sprint performance.
While the above studies provide some insight into the relationship between sleep deprivation and performance, most athletes are more likely to experience acute bouts of partial sleep deprivation in which sleep is reduced for several hours on consecutive nights .
Partial Sleep Deprivation
A small number of studies have examined the effect of partial sleep deprivation on athletic performance. Reilly and Deykin  reported decrements in a range of psychomotor functions after only 1 night of restricted sleep; however, gross motor function such as muscle strength, lung power, and endurance running were unaffected. Reilly and Hales  reported similar effects in women following partial sleep deprivation, with gross motor functions being less affected by sleep loss than tasks requiring fast reaction times.
The effect of 2.5 h of sleep deprivation per night over 4 nights was measured in eight swimmers . No effect of sleep loss was observed when investigating back and grip strength, lung function, or swimming performance. However, mood state was significantly altered with increases in depression, tension, confusion, fatigue, and anger, and decreases in vigour. Reilly and Piercy  found a significant effect of sleep loss on maximal bench press, leg press, and dead lifts, but not maximal bicep curl. Submaximal performance was, however, significantly affected on all four tasks and to a greater degree than maximal efforts. The greatest impairments were found later in the protocol, suggesting a cumulative effect of fatigue from sleep loss .
From the available research it appears that submaximal prolonged tasks may be more affected than maximal efforts, particularly after the first 2 nights of partial sleep deprivation . While partial sleep deprivation may be common in the elite athlete, much of the research in this area has not examined the elite athlete, and the degree of sleep deprivation in those studies may be more than experienced during normal training phases. In addition, understanding the sleep deprivation that may occur during competition phases and how this affects performance is an important area of future research.
Effects of Sleep Extension
Another means of examining the effect of sleep on performance is to extend the amount of sleep an athlete receives and determine the effects on subsequent performance. Mah et al.  instructed six basketball players to obtain as much extra sleep as possible following 2 weeks of normal sleep habits. Faster sprint times and increased free-throw accuracy were observed at the end of the sleep extension period. Mood was also significantly improved, with increased vigour and decreased fatigue . The same research group also increased the sleep time of swimmers from their usual sleep amount to 10 h per night for 6–7 weeks. Following this period, 15 m sprint, reaction time, turn time, and mood all improved .
Effects of Napping
Athletes suffering from some degree of sleep loss may benefit from a brief nap, particularly if a training session is to be completed in the afternoon or evening. Waterhouse et al.  investigated the effects of a lunchtime nap on sprint performance following partial sleep deprivation (4 h of sleep). Following a 30-min nap, 20 m sprint performance was increased, alertness was increased, and sleepiness was decreased when compared with the no-nap trial. In terms of cognitive performance, sleep supplementation in the form of napping has been shown to have a positive influence on cognitive tasks following a night of sleep deprivation (2 h) . Naps can markedly reduce sleepiness and can be beneficial when learning skills, strategy or tactics . Napping may also be beneficial for athletes who have to wake early routinely for training or competition and those who are experiencing sleep deprivation .
The data from these small number of studies on sleep extension and napping suggest that increasing the amount of sleep an athlete receives may significantly enhance performance. Given that recent data suggest that elite athletes have a lower total sleep time and lower sleep efficiency than non-athletes , it appears logical that increasing the quantity of sleep obtained may result in an increase in athletic performance.
Other Consequences of Sleep Deprivation
There are numerous other consequences of sleep deprivation, in addition to reduced exercise performance, that have been investigated in the general population. While those studies have not focused on athletes, some of the changes that occur in cognition, pain perception, immunity and inflammation, and metabolism and endocrine function may be relevant to the elite athlete. While it is beyond the scope of this review to examine research in each of these areas comprehensively, a summary is provided below and reference made to review articles where appropriate.
Sleep research has identified correlations between various sleep parameters (sleep architecture and efficiency) and attention, concentration, skills, perceptual function, language, memory, and executive and intellectual function . One of the most studied areas of sleep deprivation research relates to effects on alertness and performance . In this instance ‘performance’ is generally defined as goal-directed behaviour requiring mental effort. Performance deficits due to sleep deprivation are well acknowledged and understood, and it is estimated that the consequences cost billions of dollars worldwide per year due to accidents, direct healthcare costs, and reduced efficiency and productivity . Learning and memory deficits are also evident after sleep deprivation. It appears that sleep is important not only following learning for consolidation of memory, but also for preparing the brain for next-day memory formation .
Adequate cognitive function plays a key role in many sports, especially team sports. Likewise, the ability to consolidate skill memory is important for skill performance, and thus sleep deprivation may have a negative impact on performance by reducing cognitive function. While there is an absence of scientific data to date examining the role of sleep deprivation on the risk of acute injury due to decreased concentration, poor execution, or reduced reaction times, it is possible that sleep deprivation may result in an increased predisposition to injury.
It is well accepted that individuals with chronic pain frequently report disturbed sleep (changes in continuity of sleep as well as sleep architecture). However, there is also recent evidence suggesting that sleep deprivation may cause or modulate acute and chronic pain . Sleep deprivation may thus enhance or cause pain, and pain may disturb sleep by inducing arousals during sleep. A cycle may then eventuate, starting with either pain or sleep deprivation, with these two issues maintaining or augmenting each other .
Athletes may experience pain as a result of training, competition and/or injury. Evidence, although minimal at this stage, suggests that athletes may also have lower sleep quality and quantity than the general population . Therefore, appropriate pain management as well as adequate sleep is likely to be very important for athletes from both a pain and sleep perspective.
Immunity and Inflammation
Similar to the evidence regarding sleep and pain, a bidirectional relationship has also been proposed regarding sleep and immunity. Increasing amounts of evidence suggest that sleep deprivation can have detrimental effects on immune function, and that immune responses feed back on sleep architecture . In a recent review examining the link between sleep and immunity, it was concluded that sleep improves immune responses and that most immune cells have their peak pro-inflammatory activity at night . Disruptions in endocrine and physiological circadian rhythms due to sleep deprivation may result in impaired immune responses, giving rise to an increased risk of illness.
A recent study examined immune function in participants who naturally slept for short (<7 h), normal (7–9 h) or long (>9 h) durations on the night before testing . Short sleep duration was associated with 49 % higher T-cell function in response to an antigen and 30 % lower natural killer cell activity when compared with normal sleep. While the implication of high T-cell activity is unclear, it was suggested that this may be related to autoimmune diseases or low-grade systemic inflammation .
Increasing sleep duration (from 8 to 10 h) or napping (30 min) following 1 night of sleep deprivation (only 2 h of sleep) both resulted in a return of leukocyte values to normal ranges . Saliva cortisol has also been shown to decrease immediately after a nap .
Markers of the acute inflammatory system, such as interleukin (IL)-1β, tumour necrosis factor-α, IL-6, and C-reactive protein are all influenced following the manipulation of sleep . In addition, patients with insomnia and sleep apnoea have elevated inflammatory markers . This increased systemic inflammation may play an important role in homeostatic functions such as insulin sensitivity and metabolism, blood pressure and, of course, sleep itself.
From the current evidence indicating that very high levels of physical activity may have implications for the immune system , short amounts of sleep may reduce immune function, and extended sleep or napping may improve immune function, it appears that poor sleep and the resultant decreased immunity may have a negative influence on athletic performance.
Metabolism and Endocrine Function
Both laboratory and epidemiological studies support the notion that chronic partial sleep loss can increase the risk of obesity and diabetes . Potential mechanisms include changes in glucose regulation by insulin resistance, dysregulation of neuroendocrine control of appetite and/or increased energy intake [42, 43] (Fig. 2).
The exact mechanism by which decreased sleep influences glucose metabolism is thought to be multifactorial and includes decreased brain glucose utilization, alterations in sympathovagal balance, increased evening cortisol, extended night-time growth hormone secretion, and pro-inflammatory processes .
Leptin and ghrelin are hormonal regulators of food intake, with leptin exerting inhibitory effects on food intake and ghrelin being an appetite-stimulating hormone . Several studies have shown that sleep deprivation results in decreases in leptin and increases in ghrelin . Sleep restriction has also been shown to increase hunger and appetite, especially relating to carbohydrate-rich foods .
In addition to alterations in appetite-regulating hormones, the function of two major neuroendocrine axes are also negatively affected (hypothalamic-pituitary-adrenal axis and hypothalamic-pituitary-gonadal axis) . This results in both increases in the secretion of catabolic hormones such as cortisol, and changes in the secretion of anabolic hormones such as testosterone and insulin-like growth factor 1. It has been proposed that these changes in hormone patterns may reduce protein synthesis and/or increase proteolysis, thereby impairing muscle recovery .
When transitioning from wakefulness to sleep, there is a shift of autonomic balance to that of parasympathetic dominance . Therefore, sleep is associated with a decrease in both sympathetic activity and catecholamine levels, and sleep loss is associated with an increase in these variables . Sleep deprivation has also been shown to have a negative influence on the responsiveness of adrenocorticotropic hormone, adrenaline, noradrenaline, and serotonin (5-HT) receptor sensitivity . Over time, this may lead to altered stress system responsiveness, similar to that seen in mood disorders.
Chronic partial sleep deprivation in athletes may result in altered glucose metabolism and neuroendocrine function, causing concern regarding carbohydrate metabolism, appetite, food intake, and protein synthesis. These factors can all influence an athlete’s nutritional, metabolic, and endocrine status negatively and hence potentially reduce athletic performance.