The effects of a single oral dose of clonidine on morning nap sleep and daytime sleepiness were evaluated in 18 normal young adult male volunteers aged 18-21 years. Polysomnography and subjective sleepiness (Stanford Sleepiness Scale and linear analog sleepiness rating scale) measures were obtained on 2 mornings. Half the subjects received placebo and half clonidine (0.25-0.3 mg) on both occasions. Subjects were instructed to stay awake on the first morning (wake) and to sleep on the second (sleep). Efforts were made to help subjects maintain arousal on the wake day. Results from the wake morning showed that clonidine subjects were significantly sleepier than placebo subjects as measured by introspection. In addition, clonidine subjects tended to have more polysomnographic signs of sleepiness (microsleeps) when not actively aroused. On the sleep morning, clonidine and placebo subjects slept for approximately 90% of the 3-h nap. Stage 1 and rapid-eye-movement (REM) sleep were significantly reduced and stage 2 sleep significantly increased in the clonidine group. In conclusion, a morning dose of clonidine produced profound sedation in waking subjects and marked REM suppression in sleeping subjects.