Sleep and recovery

Sleep requirement or sleep need is defined as the optimum amount of sleep required to remain alert and function throughout the day^{(Reference Chokroverty21)}. The National Sleep Foundation has produced guidelines regarding sleep duration recommendations^{(Reference Hirshkowitz, Whiton and Albert22)}. Sleep needs change over the lifespan from adolescents (recommended 8–10 h), to adults (recommended 7–9 h) and older adults (7–8 h)^{(Reference Hirshkowitz, Whiton and Albert22)}, however due to the nature of training and competition athletes may require more sleep. A recent study in elite athletes (n = 175) included a self-report assessment of sleep need, athletes reported an average sleep need of 8.3 ± 0.9 h^{(Reference Sargent, Lastella and Halson23)}. The length of sleep depends on number of factors, not least volitional control (e.g. staying up late, waking by alarm, socialising, etc.), which can make it difficult to characterise a ‘normal’ sleep pattern due to high individual variation^{(Reference Kryger, Roth and Dement24)}. Sleep length is also dictated by genetic determinants^{(Reference Lassi and Tucci25)}. The relationship between sleep and recovery can be viewed in terms of three key factors that affect the recuperative outcome:

1. 1. Sleep duration (total sleep requirement; hours/night, plus naps)

2. 2. Sleep quality (sleep disorders, environmental disturbance or sleep fragmentation)

3. 3. Sleep phase (circadian timing of sleep)^{(Reference Samuels, James and Lawson17)}

Muscle fatigue or soreness may adversely affect sleep, with inflammatory cytokines linked to disruption of normal sleep^{(Reference Hausswirth, Louis and Aubry26,Reference Imeri and Opp27)} , while poor sleep increases muscle soreness^{(Reference Hagenauer, Crodelle and Piltz28)}. Inadequate recovery can reduce autonomic nervous system (ANS) resources, with an associated reduction in heart rate variability (HRV) and increased heart rate^{(Reference Hynynen, Uusitalo and Konttinen29)}. HRV responds to changes in training load and is negatively affected by total sleep deprivation^{(Reference Zhong, Hilton and Gates30)}, following adequate recovery, HRV values increase due to a slower heart rate and reduced ANS excitability.

Athletes may experience significant problems sleeping due to lack of an appropriate sleep routine relating to changing training schedules, timetables and other sleep-incompatible behaviours, e.g. late night blue light exposure^{(Reference Tuomilehto, Vuorinen and Penttilä31)}. For athletes, post competition routines and heightened arousal (i.e. medical care, recovery strategies, meals, media commitments and travel) can lead to later bedtimes, which can adversely affect sleep quality and quantity. Reduced sleep is associated with increased catabolic and reduced anabolic hormones which results in impaired muscle protein synthesis^{(Reference Fullagar and Bartlett32)}, blunting training adaptations and recovery. Extensive sleep loss (≥30 h sleep deprivation) has been associated with a reduction in muscle glycogen content^{(Reference Skein, Duffield, Edge, Short and Mundel33)}.

Sleep was reported as the most important recovery modality utilised by South African athletes (n = 890; International n = 183, National n = 474, Club n = 233) [15]. While Erlacher et al.^{(Reference Erlacher, Ehrlenspiel and Adegbesan34)}, found that 66% (n = 416) of elite German athletes (n = 632) reported pre-competition sleep problems including difficulty falling asleep, waking during the night and early final waking times. Similarly, modest sleep loss has been associated with reduced psychomotor performance in adults as demonstrated by increased psychomotor vigilance task lapse totals^{(Reference Vgontzas, Fernandez-Mendoza and Liao35)}.

Sleep patterns have been shown to influence athletic performance while athletic performance has also been shown to impact sleep patterns. Following a single night of sleep restriction (5 h), mean tennis serve accuracy declined significantly from baseline 53% to 37%^{(Reference Reyner and Horne36)}. Tuomiletho et al.^{(Reference Tuomilehto, Vuorinen and Penttilä31)}, investigated the sleep patterns of professional male ice hockey players (n = 23) using polysomnography (PSG) and found than mean total sleep time (415 mins) was inadequate. Sleep duration (<8 h) has been identified as the strongest predictor of injury in adolescent athletes)^{(Reference Milewski, Skaggs and Bishop19)}. The Karalinska Athlete Screening Injury Prevention (KASIP) study investigated injury occurrence in Swedish adolescent elite athletes (n = 340; 178 males and 162 females) and demonstrated that athletes reaching the National Sleep Foundation^{(Reference Hirshkowitz, Whiton and Albert37)} sleep guidelines (>8 h) reduced injury risk by 61%) while athletes who consumed the recommended nutrition guidelines reduced injury risk by 64%)^{(Reference von Rosen, Frohm and Kottorp18)}. These findings illustrate the interactions between sleep and athlete recovery. Sleep extension (>10 h per night for 2 weeks) demonstrated improved sprint time (16.2 vs 15.5 sec), free throw shooting accuracy (7.9 vs 8.8), 3-point shooting accuracy (10.2 vs 11.6), mean reaction time (psychomotor vigilance task 310.84 ± 77.13 vs 274.51 ± 42.01ms) in collegiate Basketball players (n = 11)^{(Reference Mah, Mah and Kezirian38)}. Profile of mood states (POMS) scores indicated physical (7.8 vs 8.8) and mental (6.9 vs 8.8) wellbeing in training and games also improved following the period of sleep extension^{(Reference Mah, Mah and Kezirian38)}, however, the absence of a control group must be noted as it is a limitation of this study. More research is necessary to investigate the sleep of athletes and potential interventions to improve overall sleep quality and quantity.

Caffeine

Caffeine increases the state of alertness, antagonising adenosine receptors, which also leads to a reduction in the inclination to sleep^{(Reference Close, Hamilton and Philip3,Reference Foster44)} . Caffeine consumption can lead to poor sleep which, in turn, can lead to increased caffeine consumption^{(Reference Del Coso, Muñoz and Muñoz-Guerra45)}.

It has been suggested that caffeine use is commonplace in athletic populations; 75–90% of athletes consume caffeine before or during competition^{(Reference Del Coso, Muñoz and Muñoz-Guerra45–Reference Desbrow and Leveritt47)}. In endurance athletes (n = 234) higher levels of consumption of caffeinated beverages has been linked to poorer subjective sleep quality^{(Reference Moss, Zhang and Kreutzer48)}. While, it has been suggested that chronic low dose caffeine ingestion may blunt any potential ergogenic effects^{(Reference Beaumont, Cordery and Funnell49)}, moderate doses (∼3 mg/kg/d) appear to pose no problems for most athletes^{(Reference Pickering and Kiely50)}. However, in terms of sleep, moderate caffeine doses have been shown to increase sleep onset latency (i.e. the amount of time it takes to get to sleep) and decrease total sleep time and sleep efficiency^{(Reference Miller, O’Connor and Orr51)}. Hence, athletes training/competing in the late afternoon (>5 pm) need to consider its potentially detrimental effect on sleep. It has recently been suggested that athletes should adopt a strategic individualised approach to caffeine consumption during competition^{(Reference Dunican, Higgins and Jones52)}. An afternoon nap has been suggested as an effective strategy for athletes to attenuate the cognitive and physical deterioration in performance resulting from either sleep loss or fatigue induced by training/competition^{(Reference Daaloul, Souissi and Davenne53)}. Equally, naps have been shown to enhance mood, alertness and cognitive performance in those who typically get the amount of sleep they need on a nightly basis^{(Reference Milner and Cote54)}, therefore napping may be an effective strategy even for athletes who get adequate sleep. In terms of duration, it has been suggested that when athletes have a nap opportunity, <30 mins is preferrable to avoid sleep inertia (i.e. a feeling of disorientation/drowsiness upon waking)^{(Reference Walsh, Halson and Sargent42)}. A 15–20 min ‘coffee-nap’ mid-afternoon has also been proposed whereby athletes consume caffeine (150–200 mg) immediately before napping to counterbalance sleepiness following the nap^{(Reference Ordóñez, Oliver and Bastos43,Reference Hayashi, Masuda and Hori55)} . Alternatively, 90 mins is also considered an optimal nap period as this facilitates a complete sleep cycle to occur, reducing the effects of sleep inertia^{(Reference Davies, Graham and Chow56)}.

Carbohydrate

The majority of the research has focused on high glycaemic index (GI) carbohydrate consumption pre-sleep. Carbohydrate consumption has been demonstrated to increase plasma tryptophan concentrations^{(Reference Afaghi, O’Connor and Chow57)}, increasing the tryptophan:LNAA ratio in the blood and may compliment the sleep promoting effect of tryptophan rich protein^{(Reference Peukhuri, Sihvola and Korpela58)}, this increases tryptophan availability for synthesis to serotonin and ultimately melatonin^{(Reference Doherty, Madigan and Warrington59)}. A high GI evening meal (4 hours before bed) reduced sleep onset latency (9.0 ± 6.2 mins) compared to a low GI meal (17.5 ± 6.2 min) and the same meal consumed 1 hour before bed (14.6 ± 9.9 min)^{(Reference Foster44)}. Lower carbohydrate intake has been associated with insomnia symptoms (difficulty maintaining sleep)^{(Reference Afaghi, O’Connor and Chow57)}. Higher consumption of wholegrains has been associated with lower risk of sleep problem in endurance athletes^{(Reference Moss, Zhang and Kreutzer60)}. Consumption of a high-carbohydrate meal (130 g) when compared to a low-carbohydrate meal (47 g), or a meal containing no carbohydrate, 45 min before bedtime increased REM and decreased light sleep and wakefulness^{(Reference Porter and Horne61)}. The impact of carbohydrate content and timing of evening meals on sleep and athlete recovery requires further investigation.

Creatine

Creatine supplementation has been emerging as a potential option for athletes to ameliorate the decline in cognitive performance associated with sleep loss. Previous research has demonstrated that 50 mg and 100 mg/kg doses of creatine alleviate the decline in simple skill performance accuracy associated with sleep loss (3–5 h sleep)^{(Reference Cook, Crewther and Kilduff62)}. Following 24 h of sleep deprivation, creatine supplementation was shown to reduce performance deficits in reaction time, mood and balance^{(Reference Ling, Kritikos and Tiplady63)}. Creatine supplementation has also been demonstrated to attenuate the loss of complex central executive function associated with sleep deprivation^{(Reference Hammett, Wall and Edwards64)}. Recent research has suggested than a single high dose of creatine (0.35 g/kg) can partially reverse metabolic alterations and fatigue related cognitive deterioration following sleep deprivation^{(Reference Gordji-Nejad, Matusch and Kleedörfer65)}.

Protein

Dietary sources of tryptophan (e.g. milk, turkey, chicken, fish, eggs, pumpkin seeds, beans, peanuts, cheese and leafy green vegetables) has been shown to improve sleep. In a comparison of the effect on sleep of tryptophan-enriched muesli bars plus glucose with bars containing 250 mg pharmaceutical tryptophan plus glucose and glucose alone^{(Reference Hudson, Hudson and Hecht66)}. The muesli bars and the pharmaceutical dose produced similar results (5.5% and 6.5% respectively) for reduction of time awake during the night^{(Reference Hudson, Hudson and Hecht66)}, indicating that relatively small doses (250 mg) of dietary tryptophan can positively impact sleep. Tryptophan depletion studies have demonstrated that decreased tryptophan plasma concentrations result in increased sleep fragmentation^{(Reference Arnulf, Quintin and Alvarez67,Reference Bhatti, Gillin and Seifritz68)} .

Athletes routinely consume protein to facilitate recovery and support immune function^{(Reference Gratwicke, Miles and Pyne69)}. While research is emerging supporting pre-sleep protein ingestion for muscle recovery^{(Reference Snijders, Trommelen and Kouw70,Reference Falkenberg, Aisbett and Lastella71)} , the impact of pre-sleep ingestion of doses of whey and/or casein warrants further investigation with regards both muscle recovery and sleep improvement.

Melatonin

In humans melatonin it is the final product in the metabolism of the amino acid tryptophan and is secreted by the pineal gland at the onset of darkness, triggering sleep due it its hypothermic effect^{(Reference Halson72)}. Ingestion of melatonin affects sleep propensity and has hypnotic effects enhancing sleep quality and duration^{(Reference Brzezinski73)}, pharmacological melatonin is commonly used in athletes to manipulate sleep patterns. A positive effect of dosages of either 0.3mg or 1mg of exogenous melatonin on sleep latency have been observed, when administered between 6:00pm and 8:00pm^{(Reference Pires, Benedito-Silva and Pinto74)}. However, the impact was time dependent as the 0.3mg dose increased sleep onset latency and there was no effect when the 1mg dose was administered at 9:00pm^{(Reference Pires, Benedito-Silva and Pinto74)}. A dose response relationship was not evident as the 0.3 mg dose, which is similar to endogenous melatonin concentrations, was as effective as the 1mg dose when administered between 6:00pm and 8:00pm.

Tart cherries contain high concentrations of melatonin. Significantly reduced insomnia severity index scores (13.2 ± 2.8 versus control 14.9 ± 3.6) and wake after sleep onset (WASO i.e. time awake) (62.1 ± 37.4 min versus control 79.1 ± 38.6), was observed in older adults following consumption of a tart cherry juice blend, compared to a placebo^{(Reference Pigeon, Carr and Gorman75)}. Research was conducted to investigate if melatonin is the mechanism of tart cherry juice (2 × servings of 30mls concentrate) sleep enhancement and improved sleep time and quality^{(Reference Howatson, Bell and Tallent76)}. Total Melatonin content was significantly elevated and significant increases in time in bed (+24 minutes), total sleep time (+34 minutes) and sleep efficiency total (82.3%) and a significant reduction in daytime napping (−22%) were associated with cherry juice supplementation^{(Reference Howatson, Bell and Tallent76)}. Although no difference was observed in timing of the circadian rhythm, there was a trend to a higher mesor and amplitude. The range of phenolic compounds in cherries which have anti-inflammatory and antioxidant properties may enhance post exercise recovery as well as sleep^{(Reference McHugh77)}.

Magnesium

Magnesium is believed to enhance melatonin secretion and act as a GABA agonist, the main inhibitory neurotransmitter that acts on the central nervous system^{(Reference Peukhuri, Sihvola and Korpela58)}. It has been noted that deficiencies in B vitamins and magnesium may also disrupt sleep^{(Reference Peukhuri, Sihvola and Korpela58)}. Magnesium supplementation has been associated with reductions in daytime sleepiness, sleep onset latency and insomnia symptoms and increased total sleep time and sleep efficiency in older adults^{(Reference Cao, Zhen and Taylor78–Reference Zhang, Chen and Lu80)}.

Kiwifruit

The chemical composition of kiwifruit is of considerable interest in terms of nutritional value and health benefits. The composition of kiwifruit varies depending on multiple factors such as horticulture, region, soil type, storage, ripening condition and maturity of the fruit^{(Reference Drummond81)}. Interest in the antioxidant capacity, enzyme, polyphenolic and phytochemical content of kiwifruit has increased steadily over the last decade. It has been suggested that the various bioactive components in kiwifruit may act synergistically affecting various physiological and metabolic^{(Reference Singletary82)}. Contemporary research has focused on the health benefits of kiwifruit particularly in relation to antioxidant capacity, digestion, iron nutrition, metabolic health and immune function^{(Reference Singletary82)}. Kiwifruit contain significant amounts of vitamin C but also contain a range of other health promoting nutrients such as vitamin E, vitamin K, folate, beta-carotene, lutein, potassium, copper and fibre^{(Reference Stonehouse, Gammon and Beck83)}.

Kiwifruit are nutritionally dense containing a range of nutrients that can benefit sleep and recovery including serotonin, vitamin C, vitamin E, folate, anthocyanidins and carotenoids^{(Reference Lin, Tsai and Fang84)}. A study involving healthy adult volunteers (n = 25) who self-reported sleep disturbance demonstrated consumption of 2 kiwifruit, 1 hour before bedtime for 4 weeks significantly improved actigraphy measured total sleep time (16.9%) and sleep efficiency (2.4%)^{(Reference Lin, Tsai and Fang84)}. Self-report measures also improved significantly, wake time after sleep onset reduced (−28.9%), sleep latency reduced (−35.4%) while sleep efficiency increased (5.4%)^{(Reference Lin, Tsai and Fang84)}. It is clear that sleep quality was significantly improved following the 4-week kiwifruit intervention. In a similar study, students (n = 74) with diagnosed insomnia consumed either 130 g of kiwifruit or a placebo (130 g pear), 1 hour before bed for 4 weeks. While there were no statistically significant differences in objective measures of sleep, there were statistically significant group x time effects for subjective sleep quality and daytime function^{(Reference Nødtvedt, Hansen and Bjorvatn85)}. Kiwifruit consumption (2 × kiwifruit 1 hour before bed) has also been shown to improve subjective sleep and recovery in elite athletes, from baseline to post-intervention (4 weeks) there were clinically significant improvements in sleep quality (i.e. improved Pittsburgh Sleep Quality Index (PSQI) global scores and sleep quality component scores) and improvements in recovery stress balance (i.e. reduced general stress and sports stress scales)^{(Reference Doherty, Madigan and Nevill86)}. During the intervention significant increases in total sleep time and sleep efficiency % and significant reductions in number of awakenings and WASO were observed^{(Reference Doherty, Madigan and Nevill86)}.

The Serotonin content in kiwifruit may contribute to improved sleep while the rich antioxidant content may supress free radical expression and inflammatory cytokines. Folate deficiency has been linked to insomnia and restless leg syndrome, the folate in kiwifruit may improve folate status and consequently improve sleep^{(Reference Lin, Tsai and Fang84)}. Although folates are widely consumed in the diet, they are destroyed by cooking or processing. Further research is necessary to investigate the potential benefits and practical application of kiwifruit supplementation to promote post-exercise recovery and promote or improve sleep in athletes.

Practical applications

Based on the available evidence some practical recommendation can be made to promote athletes’ sleep and recovery which are summarised in Table 1.

Table 1.

Summary of potential nutrition strategies for athletes to improve/promote sleep or mitigate sleep loss

Adapted from Ref.^{(Reference Doherty, Madigan and Warrington59)}.