The effect of an aqueous extract of valerian (Valeriana officinalis L.) root on subjectively rated sleep measures was studied on 128 people. Each person received 9 samples to test (3 containing placebo, 3 containing 400 mg valerian extract and 3 containing a proprietary over-the-counter valerian preparation). The samples, identified only by a code number, and presented in random order, were taken on non-consecutive nights. Valerian produced a significant decrease in subjectively evaluated sleep latency scores and a significant improvement in sleep quality: the latter was most notable among people who considered themselves poor or irregular sleepers, smokers, and people who thought they normally had long sleep latencies. Night awakenings, dream recall and somnolence the next morning were relatively unaffected by valerian. With the proprietary valerian-containing preparation, the only change was a significant increase in reports of feeling more sleepy than normal the next morning. Thus the questionnaire, simple to use and non-invasive, provides a sensitive means for detecting the effects of mild sedatives on different aspects of sleep in man. It also allows identification within the test population of the subgroups most affected.