*4.1. Carbohydrate*

Carbohydrates have long been utilized in sport to fuel athletes who engage in intensified and prolonged exercise [2,10]. The traditional focus of carbohydrate supplementation is often on restoring muscle and liver glycogen levels between training sessions or matches [40]. While evidence for the amount, type, and timing of carbohydrate intake for recovery is well documented, some studies have also explored the use of carbohydrate for promoting sleep.

Most studies have focused on the effects of high and low glycemic index (GI) carbohydrate feeding [22,30]. Evidence from studies in athlete samples is limited and equivocal, with one study in Brazilian male basketballers reporting a non-significant increase in TST and decrease in SOL, but also reductions in SE and an increase in WASO [22]. However, studies in healthy populations are more definitive, with consistent reports of improvements in sleep following high GI carbohydrate feeding [25,30]. In a study of particular relevance to athletes, Vlahoyiannis and colleagues [30] provided healthy, physically active young males with either a high or low GI meal immediately following an evening bout of intermittent sprint exercise. While there were no effects of meal GI on sleep architecture (i.e., proportion of sleep time spent in different sleep stages), the high GI meal substantially improved TST, SE, SOL, and WASO. As such, high GI feeding may ameliorate the sleep disturbances experienced by athletes following evening training and competition.

The timing of a high GI meal may influence its subsequent impact on sleep. One study reported that SOL was longer and subjective sleepiness was lower when a high GI meal was ingested 1 h before bed rather than 4 h before bed [25]. The proposed mechanism for improvement in sleep following a high GI meal is an increase in the plasma ratio of tryptophan to large neutral amino acids (TRP/LNAA). Tryptophan is an essential amino acid and serves as a precursor to the synthesis of serotonin and melatonin [41], both important regulators of sleep. By increasing the TRP/LNAA, tryptophan can more readily cross the blood brain barrier, consequently leading to an increase in the synthesis of serotonin [42] and then downstream increases in secretion of melatonin [25,30]. Importantly, it appears that the TRP/LNAA peaks 2–4 h after ingestion of a high carbohydrate meal

with minimal change in the first 1–2 h [43]. Therefore, high GI feeding should occur at least two hours before bedtime to maximize the potential impact on sleep.

In addition to manipulating glycemic load (which combines both the quality (GI) and the quantity of carbohydrates), studies have investigated the effects of restricting carbohydrate intake on sleep parameters [26]. Ingestion of a very low carbohydrate meal 4 h before bed increased the proportion of slow-wave sleep assessed by polysomnography (PSG), the gold standard of sleep assessment [26]. An increase in slow-wave sleep could be particularly beneficial to athletes as it is thought to play an important role in performance recovery [44] and assists in energy conservation and recuperation of the nervous system [1,45]. However, athletes also need to consider the impact of low carbohydrate feeding on muscle and liver glycogen restoration, and decisions on priorities for refueling versus sleep promotion will need to be made.

The effects of different evening carbohydrate meals on sleep remains somewhat unclear given the heterogeneity of current research findings. Further research targeting the ideal timing, quantity, and source of carbohydrate ingestion, as well as its interaction with other macro and micronutrients is needed in an athletic population to determine its efficacy. More definitive guidelines on carbohydrate feeding to improve athlete sleep can then be developed.
