*4.2. Protein*

Protein plays an important multifactorial role in an athlete's training and competition cycle by facilitating muscle repair and remodeling and supporting adequate immune function [2,46,47]. Whey protein supplementation post-exercise and pre-sleep has been introduced to enhance whole body protein synthesis and muscle performance during overnight recovery [47]. A specific whey protein which has recently been investigated as a nutritional pre-sleep intervention is α-lactalbumin [42,48]. α-Lactalbumin is reported to have the highest natural level of tryptophan in protein food sources [48]. Tryptophan is an essential amino acid and serves as a precursor to the synthesis of serotonin and melatonin, both of which are involved in the regulation of sleep [41].

The initial evidence demonstrating the potential effectiveness of α-lactalbumin on sleep was obtained from a series of studies conducted in stress-vulnerable participants with and without sleep complaints [49,50]. Although not directly investigating sleep parameters, sleep quality may have improved with a substantial reduction in sleepiness, and improved attention processes the morning following the intervention. In addition, these studies observed an increase in TRP/LNAA, which is essential for serotonin synthesis in the brain. Another study also reported decreases in depressive feelings under stress in stressvulnerable participants consuming α-lactalbumin [50]. There are strong links between sleep and feelings of anxiety and depression, with women experiencing more insomnia complaints than males [51]. Supplementation with α-lactalbumin that can increase sleep quality and decrease depressive feelings may be beneficial during high-stress team-sport seasons, particularly for female athletes.

Recently, Ong and colleagues investigated the efficacy of an α-lactalbumin treatment (20 g, 1 h prior to bed) in healthy male subjects with no known sleep conditions or impairments [28]. Sleep quantity was increased as was objective (13%; via actigraphy-based assessment) and subjective (11%; via sleep diary) TST compared to placebo, as well as 7% higher objective SE [28]. In comparison, there was no difference for any actigraphyrecorded sleep variables between α-lactalbumin and collagen peptide supplementation in a small cohort of cyclists [24]. While there is preliminary evidence to support the efficacy of α-lactalbumin to improve sleep, further well-designed studies are required to confirm its effectiveness. Specifically, research examining the use of α-lactalbumin chronically in an athletic population is warranted. To date, limited studies involving night-time protein ingestion have been carried out for longer than four weeks in the general population. There is currently no evidence for the efficacy of nutritional interventions when used chronically, or in a field setting in an athletic population during training and competition.
