Sleep: A Brief Review of its Impact on Sports Performance
Although research is still not able to fully explain the causal mechanisms, trainers, coaches and athletes all generally recognize the importance of sleep - and that lack of sleep is detrimental to performance. For example, in a study from the Australian Institute of Sport 1 both athletes and coaches ranked sleep as the most prominent problem when they were asked about their perceptions of the causes of fatigue and tiredness. What research has determined2 is that lack of sleep (i) increases levels of hormones such as cortisol, which breaks down body/muscle tissue and, (ii) reduces levels of hormones such as testosterone, which builds up body/muscle tissue. When these processes occur, they can trigger loss of muscle mass as well as slowed injury repair and tissue recovery, which can delay a return to training or competition. Compromised sleep has also been reported3 as negatively influencing learning, memory, cognition, pain perception and inflammation. All these factors can ultimately impact an athlete’s nutritional, metabolic and endocrine status.
Impact of Sleep Deprivation on Muscle Fuel, athletes and athletic performance
One of the most immediate and consequential results of sleep deprivation is a reduction in muscle fuel/glycogen. A recent study 4 conducted with male athletes during simulated team-sport exercise bouts, reported that 30 hours of sleep loss (vs no sleep loss) was associated with reductions in muscle glycogen and increases in perceptions of stress. This, in turn, reduced performance and pacing strategies during intermittent sprint exercises. Such decrements can also lead to an earlier onset of fatigue, with potential increases for soft tissue injury risk in later stages of the game.
Sleep deprivation (~24-48hrs) has also been reported to negatively affect performance outcomes including maximal, peak and mean power in cycling 5, vertical jump performance6, knee extension/flexion peak torque6,7, and running performance8. Sleep deprivation is also associated with negative psychological effects: following 24 hours of sleep deprivation (vs no sleep deprivation), mood states in college age weight lifters were significantly altered, with confusion, vigor, fatigue and total mood disturbance all negatively affected by sleep deprivation9. Studies have also found indications of an increased perception of effort with endurance exercise following sleep deprivation4,8.
Sleep has been recognized as an essential component of recovery from, and preparation for, high-intensity training. Research 10 suggests that athletes are able to overcome the effects of sleep deprivation during high intensity and/or supra maximal exercise. However, they become less motivated to maintain a high level of performance in sustained exercise or in repeated exercise bouts that commonly occur in training.
Impact of Partial Sleep Deprivation on athletes and athletic performance
The research is clear on the negative impact of chronic sleep deprivation. However, most athletes are likely to experience acute bouts of partial sleep deprivation in which sleep is reduced for several hours on consecutive nights. Restricting sleep to less than 6 h per night for 4 or more consecutive nights has been shown to impair cognitive performance and mood 11, glucose metabolism12, appetite regulation13, and immune function14. This type of evidence has led to the recommendation15 that adults should obtain 8 h of sleep per night to prevent neuro-behavioral deficits. In contrast to this, surveys have indicated that athletes may be obtaining, on average, less than 8 h of sleep per night3. Research16 has also found that submaximal, prolonged tasks may be more affected than maximal efforts, particularly after the first 2 nights of partial sleep deprivation.
Factors impacting sleep deprivation/disturbances
Sleep disturbances in athletes can occur either before important competitions, or during normal training 3. Before competitions, athletes have reported disturbed sleep because of factors including thoughts or nervousness about the competition, unusual sleep surroundings and noise in the bedroom17. During normal training sleep may be due to a poor routine caused by early training sessions, poor sleep habits (i.e. watching television in bed), caffeine use, and excessive worrying or planning. Some anecdotal evidence also suggests that athletes who compete at night have significant difficulties falling asleep post-competition3. Some of these factors are able to be improved with selective changes in lifestyle behaviors
While the importance of sleep in human functioning has typically focused on deprivation, a growing body of research has investigated the impact on performance of increasing daily sleep time (“Sleep Extension”). A recent study 18 in healthy college basketball players who extended their sleep time by approximately 30% over a 5 to 7 week period, showed significant improvements in reaction times, sprint times and shooting percentage. Mood, fatigue, and vigor also improved with increased total sleep time. The study also described similar results in other sleep extension studies, as well as findings of increased vigilance, and reduced time to fall asleep (“Sleep Latency”). Notably, the authors found that athletes were not able to accurately assess how much sleep they actually obtained each night “...leading to a misperception regarding the duration of sleep that constitutes adequate nightly sleep time.”
A recent comprehensive review 3 of sleep in elite athletes - as well as nutritional interventions to enhance sleep - listed the following recommendations:
From: Halson, 2014: In the first instance, athletes should focus on utilizing good sleep hygiene to maximize sleep quality and quantity. While research is minimal and somewhat inconclusive, several practical recommendations may be suggested:
- High GI foods such as white rice, pasta, bread, and potatoes may promote sleep; however, they should be consumed more than 1 h before bedtime.
- Diets high in carbohydrate may result in shorter sleep latencies.
- Diets high in protein may result in improved sleep quality.
- Diets high in fat may negatively influence total sleep time.
- When total caloric intake is decreased, sleep quality may be disturbed.
- Small doses of tryptophan (1 g) may improve both sleep latency and sleep quality. This can be achieved by consuming approximately 300 g of turkey or approximately 200 g of pumpkin seeds.
- The hormone melatonin and foods that have a high melatonin concentration may decrease sleep onset time.
- Subjective sleep quality may be improved with the ingestion of the herb valerian; however, as with all supplements, athletes should be aware of potential contaminants as well as the inadvertent risk of a positive drug test.
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