Melatonin (MLT), a pineal hormone, has some sedative and hypnotic properties. To explore this effect further 20 young, healthy volunteers exposed to artificial insomnia participated in a double-blind, placebo controlled, parallel group design study. They slept in a sleep laboratory for several consecutive nights and were polygraphically monitored and subjected to a battery of psychometric tests and standardized self-report questionnaires each morning. One night all subjects received only placebo (21:00 hours) and on a second night half of them were subjected to placebo and half to MLT. On the later night blood MLT levels were measured. Polygraphic recordings revealed that MLT at bedtime decreased the time the subjects were awake before sleep onset (P<0.025), sleep latency (P<0.05), and the number of awakenings during the total sleep period (P<0.025), and increased sleep efficiency (P<0.05). In addition, it decreased sleep stage 1 (P<0.05) and increased sleep stage 2 (P<0.025). On the morning following the treatment most objective and subjective measures for awakening quality showed a trend towards improvement after MLT. One hour after its oral administration, serum MLT rose to a high pharmacological level (25817 pg/ml; median), but individual peak serum MLT levels varied by a factor of 300. From the data collected it is concluded that a single pharmacological dose of MLT exerts a hypnotic effect by accelerating sleep initiation, improving sleep maintenance and altering sleep architecture in a similar manner to anxiolytic sedatives; objective and subjective measures for awakening qualitiy indicate good tolerance of one dose of MLT without hangover problems on the following morning. Oral administration of crystalline MLT, however, results in high interindividual differences in absorption.