The effects of 3 g L-tryptophan on sleep, performance, arousal threshold, and brain electrical activity during sleep were assessed in 20 male, chronic sleep-onset insomniacs (mean age 20.3 +/- 2.4 years). Following a sleep laboratory screening night, all subjects received placebo for 3 consecutive nights (single-blind), ten subjects received L-tryptophan, and ten received placebo for 6 nights (double-blind). All subjects received placebo on 2 withdrawal nights (single-blind). There was no effect of L-tryptophan on sleep latency during the first 3 nights of administration. On nights 4-6 of administration, sleep latency was significantly reduced. Unlike benzodiazepine hypnotics, L-tryptophan did not alter sleep stages, impair performance, elevate arousal threshold, or alter brain electrical activity during sleep.