Eighteen female subjects with demonstrated laboratory sleep onset latency greater than 20 minutes for two nights participated in this double blind study of the effectiveness of l-tryptophan as a hypnotic. Standard sleep recordings were made on 10 nights over a 3 month period with lights out occurring 20 minutes after drug administration (placebo, 1 gm. l-tryptophan, 3 gms. l-tryptophan). Neither dose of l-tryptophan differed from placebo as to the amount of REM, SWS or wakefulness, but 3 gms. significantly reduced sleep onset latency on some of the nights. Those subjects with latencies longer than 40 mins. had the greatest reduction in latency with 3 gms. and also evidenced high levels of anxiety on the Taylor Manifest Anxiety Scale initially. Subjects with latency between 20 and 40 minutes appeared to receive the longest lasting hypnotic effect from the higher dose. Adaptation to the sleep lab took place across the entire 10 nights of the study. Therefore, valid comparisons between treatments in a sleep study extending over a number of nights should be made between temporally adjacent samples.