Sleep is generally recognised as a critical factor in athlete’s performance. Sleep is thought to affect both physiological and cognitive function, that can affect sports performance. Recent evidence, suggests that athletes have lower quality of sleep as well as lower quantity of sleep, compared with the non-athlete, particularly during periods of intensified training (Read Sleep disturbances in trained athletes). Lack of sufficient sleep is likely to have detrimental effects on athletic performance.Compromised sleep might also influence cognition, learning, memory, pain perception, immunity and inflammation. Chronic partial sleep deprivation may result in changes in carbohydrate metabolism, protein synthesis, appetite, and food intake. These factors can ultimately have a negative influence on an athlete’s nutritional, metabolic and hormonal status and could therefore potentially reduce athletic performance.
A number of neurotransmitters (e.g. 5-HT, gamma-aminobutyric acid, orexin, melanin-concentrating hormone, norepinephrine, and histamine) have been associated with the sleep-wake cycle. There are some nutritional interventions that can influence these neurotransmitters in the brain and so may thus influence sleep. For example, carbohydrate, tryptophan, valerian, and melatonin and others have been investigated as possible sleep inducers and represent promising potential interventions to improve sleep quantity and/or quality.
Synthesis of 5-HT in the brain is dependent on the availability of its precursor, the amino acid tryptophan (Trp). Trp is transported across the blood–brain barrier by a transport system that is shared by a number of large neutral amino acids (LNAA) including the branched chain amino acids (BCAA) leucine, isoleucine and valine. Thus, the ratio of Trp/LNAA in the blood is crucial to the rate of transport of Trp into the brain. Ingestion of protein generally decreases the uptake of Trp into the brain, as Trp is the least abundant amino acid and therefore other LNAA are preferentially transported into the brain. The ingestion of carbohydrate, however, increases brain Trp as the rise in circulating insulin (as a result of the increase in blood glucose concentration) stimulates the uptake of LNAA into skeletal muscle, which results in an increase in free Trp in the circulation, an effect that promotes its uptake into the brain.
There have been numerous investigations of the effects of Trp supplementation on sleep (1), and it appears that Trp doses as low as 1 g can improve sleep latency (time before falling asleep) and subjective sleep quality.
Melatonin is a hormone that influences the sleep-wake cycle by inducing a sleep promoting effect. Light exposure of the retina of the eyes results in a suppression of melatonin secretion. Some nutritional interventions that increase Trp availability or reduce the plasma concentration of LNAA can increase melatonin production and promote sleep. This can be achieved by several means:
a high protein diet that contains more Trp than LNAA
ingestion of carbohydrate (This may increase the ratio of free Trp to LNAA and facilitate the release of insulin, which promotes the uptake of BCAA into the muscle)
Research investigating the use of melatonin for primary insomnia has demonstrated inconclusive results. A meta-analysis reported a reduction in sleep-onset latency of 7 min, and concluded that while melatonin appeared safe for short-term use, there was no evidence that melatonin was effective for most primary sleep disorders (2).
Another recently investigated nutritional supplement is tart cherry juice which contains relatively large amounts of phytochemicals including melatonin. The ingestion of tart cherry juice has been shown to increase urinary melatonin, and when consumed for a one week period was shown to result in modest improvements in sleep time and quality (3) compared with placebo.
Recent studies on the effects of carbohydrate ingestion on indices of sleep quality and quantity indicate that high carbohydrate meals consumed in the hour before bedtime improve sleep quality and reduce wakefulness. Solid compared with liquid meals tend to reduce sleep onset latency (time taken to fall asleep) up to 3 h after ingestion, and a high glycemic index (GI) meal significantly improves sleep-onset latency above that with a low GI meal if consumed 4 h (but not 1 h) before bedtime. A few studies have investigated more chronic manipulations of habitual dietary intake on sleep and these have suggested that diets higher in carbohydrate result in shorter sleep-onset latencies, diets higher in protein result in fewer wake episodes and diets high in fat may negatively influence total sleep time.
Valerian is a herb that binds to gamma-aminobutyric acid type A receptors and is thought to induce a calming effect by regulation of the nervous system. Results of a meta-analysis investigating the efficacy of valerian showed a subjective improvement in sleep quality (4). While valerian is one of the more common ingredients found in supplements claiming to promote sleep, side effects such as drowsiness, dizziness and allergic reactions can be observed.
Other suggested sleep aids have not been adequately investigated and are not supported by scientific evidence: passionflower, kava, St. John’s wort, lysine, glycine, magnesium, lavender, skullcap, lemon balm, magnolia bark, and nucleotides. Many of these can be found in supplements that are claimed to improve sleep quantity and/or quality.
Current practical recommendations to improve sleep via nutritional interventions include:
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 a supplement or approximately 300 g of turkey.
The hormone melatonin and foods that have a high melatonin concentration (e.g. tart cherries) may decrease sleep onset time.
Subjective sleep quality may be improved with the ingestion of the herb valerian
It is important to note though that the research in this area is limited and more work is needed before firm conclusions can be drawn.
References
Silber BY, Schmitt JA. Effects of tryptophan loading on human cognition, mood, and sleep. Neurosci Biobehav Rev. 34(3):387-407, 2010.
Buscemi N, Vandermeer B, Hooton N, Pandya R, Tjosvold L, Hartling L, Baker G, Klassen TP, Vohra S. The efficacy and safety of exogenous melatonin for primary sleep disorders. A meta-analysis. J Gen Intern Med. 20(12):1151-8, 2005.
Howatson G, Bell PG, Tallent J, Middleton B, McHugh MP, Ellis J. Effect of tart cherry juice (Prunus cerasus) on melatonin levels and enhanced sleep quality. Eur J Nutr. 51(8):909-16, 2012.
Fernández-San-Martín MI, Masa-Font R, Palacios-Soler L, Sancho-Gómez P, Calbó-Caldentey C, Flores-Mateo G. Effectiveness of Valerian on insomnia: a meta-analysis of randomized placebo-controlled trials. Sleep Med. 11(6):505-11, 2010.
A highly recommended overview is written by Shona Halson:
