*4.4. Other Nutritional Strategies*

There are a variety of other nutritional interventions that have been explored for their ability to improve sleep in poor sleepers, including kiwifruit and herbal supplements. Kiwifruits contain a range of nutrients that potentially augmen<sup>t</sup> sleep and recovery [44], including serotonin, a known sleep promoting hormone that helps regulate REM sleep [61]. Improved sleep was reported in poor sleepers who ingested two kiwifruits an hour before bed over a four-week intervention period [32]. Marked increases in wrist actigraphy monitored TST (16.9%) and SE (2.4%) were evident, while subjective sleep diary recordings showed a substantial decrease in WASO and SOL. The improved sleep quality may be attributable to high levels of folate in kiwifruit [10,32]. Folate deficiency has been linked to insomnia and restless leg syndrome, both of which cause large sleep disruptions and can hinder the restorative quality of sleep [32,62].

Akin to kiwifruit consumption, ingestion of GABA in different quantities has yielded improvements in the sleep quality of poor sleepers or those dissatisfied with sleep [33,39]. GABA is an inhibitory neurotransmitter that is often present in food, and its receptors in the central nervous system are often targeted by pharmacological agents such as benzodiazepines for treatment of several conditions including insomnia [63]. While benzodiazepines can improve sleep quality and quantity, they are also associated with substantial side-effects, including drowsiness, lethargy, fatigue, and, in extreme cases, impaired motor coordination and addiction [63], all of which would likely impair athletic performance. Importantly, studies in humans have reported no adverse effects of pre-sleep GABA ingestion on next-day sleepiness or fatigue, which suggests it may be useful in athletes to improve sleep.

Glycine is another inhibitory neurotransmitter that has been linked to improvements in subjective and objective sleep quality for poor sleepers [36,37], but also daytime fatigue and cognitive performance in healthy adults during simulated sleep restriction [29]. Similarly, ingestion of L-serine, a glycine precursor, reportedly leads to improvements in sleep satisfaction and subjective sleep quality in adults dissatisfied with their sleep [38]. While further evidence for the efficacy of glycine and L-serine in athlete populations is required, these results sugges<sup>t</sup> both proteins may offer athletes dissatisfied with sleep issues, or facing situational sleep restriction, a means by which to improve their sleep.

Most studies included in this review have focused on the effects of single nutrients or ingredients. However, some recent studies have examined the effects of multi-nutrient supplements on sleep. In one study on the effects of a tart cherry powder-based supplement, there was a significant improvement in SOL and a tendency towards improvement in SE across seven days of supplementation in young adult poor sleepers [35]. However, chemical analysis undertaken as part of the study indicated the supplement contained no melatonin, despite being tart cherry based. Instead, the sleep improvement was likely due to the 3 g of tryptophan and 2 g of glycine, which is similar to the 3 g dose used in other studies that report sleep improvement [29,36,37], contained in the product. Another multi-nutrient sleep study determined both the most and least optimal combination of a variety of ingredients for sleep improvement [64]. Both combinations contained a mixture of ingredients linked to improvements in sleep when used in isolation including high GI carbohydrate and tryptophan. The most optimal combination led to a reduction in SOL in a group of healthy adults with good sleep. Perhaps more importantly for athletes, the least optimal combination drink led to increases in SOL relative to a placebo supplement in the same group, emphasizing the need to consider how nutrients interact to influence sleep.
