Effects of 2.5, 5.0, 7.5, and 10.0 mg zopiclone on sleep and on performance the next day were studied in 6 healthy adult males aged between 21 and 33 years. The experiment was double-blind and placebo controlled. 5.0 and 10.0 mg zopiclone decreased the amount of awake activity and drowsy (stage 1) sleep over the first 6 h of sleep, but this effect was only present for the whole sleep period with the 10.0-mg dose. The duration and percentage of stage 3 sleep were increased with 7.5 mg. The 7.5- and 10.0-mg doses increased the combined duration of stages 2, 3, and 4 sleep over the first 6 h and over the whole night. The first period of rapid eye movement sleep was delayed with 7.5 mg, and with 10.0 mg the time spent in rapid eye movement sleep was reduced during the first 6 h of sleep, but not over the whole night. The number of substitutions in the digit symbol substitution test was decreased 9 h after ingestion of 7.5 and 10.0 mg, and the number of symbols copied reduced after 10.0 mg. The clinical dose range of zopiclone is likely to be up to 7.5 mg. The latter dose provides a useful hypnotic effect with minimal residual effects the next day, whilst 5.0 mg is appropriate for those involved in skilled tasks where even the most minor changes in performance during the early part of the next day must be avoided.