The effect of chlorpromazine on nine patients with sleep disturbance was investigated. The results showed that chlorpromazine given at bedtime coincided with a marked increase in actual sleep time as exemplified by a significant decrease in intermittent wakefulness. Total REM time was increased proportionately to the increase in actual sleep. In a more extensive study in four patients, it was found that the administration of daytime chlorpromazine (100 mg) had no direct effect on sleep as compared to placebo. In contrast, bedtime chlorpromazine was associated with significant changes in various sleep parameters. That the time course of chlorpromazine administration is responsible for the differential effect on sleep is supported by the preliminary investigation with plasma chlorpromazine levels.