The effect of L-tryptophan on night-time and day-time sleep (from 14.00 h) sleep was studied in six healthy males aged between 20 and 30 years. The doses used in the night-time studies were 2, 4 and 6 g, and in the day-time studies 1, 2 and 4 g. It was not possible to establish an effect of L-tryptophan compared with placebo on night-time sleep, but analysis of the sleep measures with 4 g compared with placebo and the other doses of L-tryptophan considered together suggested reduced awakenings, increased stage 3 and an increased percentage of REM sleep. With 4 g L-tryptophan there was an increase in the duration of stage 3 of day-time sleep compared with placebo. The studies provide marginal evidence that REM sleep may be modified by L-tryptophan in man, though the evidence is somewhat stronger that SWS may be increased. The effect on REM sleep may involve circadian mechanisms. The hypnotic activity of L-tryptophan per se is limited and uncertain.