Sleeping problems and their subsequent effects on behaviour are often attributed to deficient brain serotonin activity.
Tryptophan is a serotonin precursor but brain uptake of amino acid is dependent on nutrients that influence the availability of tryptophan compared to other amino acids.
Researchers at the University of Maastricht recruited 14 healthy people with mild sleep complaints and 14 without sleeping problems for the double-blind study.
The subjects slept at the laboratory for two separate nights so that morning performance could be evaluated after an evening diet containing either tryptophan-rich alpha-lactalbumin or tryptophan-low placebo protein.
The tryptophan-rich alpha-lactalbumin has been developed by the food science research centre TNO as an alternative to tryptophan produced by fermentation. The protein has an enriched tryptophan content of 4.8 g per 100g.
The milk protein supplements increased levels of tryptophan by 130 per cent before bedtime and modestly but significantly reduced sleepiness and improved brain-sustained attention processes the following morning, write the researchers.
"Only in poor sleepers was this accompanied by improved behavioural performance," they report in this month's issue of the American Journal of Clinical Nutrition (vol 81, no 5, pp1026-1033).