The effect of an aqueous extract of valerian root on sleep was studied in two groups of healthy, young subjects. One group (N = 10) slept at home, the other (N = 8) in the sleep laboratory. Sleep was evaluated on the basis of questionnaires, self-rating scales and night-time motor activity. In addition, polygraphic sleep recordings and spectral analysis of the sleep EEG was performed in the laboratory group. Under home conditions, both doses of valerian extract (450 and 900 mg) reduced perceived sleep latency and wake time after sleep onset. Night-time motor activity was enhanced in the middle third of the night and reduced in the last third. The data suggest a dose-dependent effect. In the sleep laboratory, where only the higher dose of valerian was tested, no significant differences from placebo were obtained. However, the direction of the changes in the subjective and objective measures of sleep latency and wake time after sleep onset, as well as in night-time motor activity, corresponded to that observed under home conditions. There was no evidence for a change in sleep stages and EEG spectra. The results indicate that the aqueous valerian extract exerts a mild hypnotic action.