Sixteen patients with a major depressive disorder were allowed to take a nap at 5 A.M. after a period of total sleep deprivation. The patients were randomly assigned to biperiden or placebo treatment prior to the nap to test the hypothesis that anticholinergic medication is capable of preventing a nap-related worsening of mood. Total sleep deprivation positively influenced mood for the whole group. Contrary to expectations, the rate of nap-related relapses of mood did not differ between placebo- and biperiden-treated individuals, and biperiden did not significantly suppress rapid eye movement (REM) sleep. There was only a tendency for lower REM time after administration of biperiden compared to placebo. This unexpected result may be due to a high cholinergic tone in the patient group investigated and a high REM propensity in the early morning hours. Studies with more selective M1/M2 receptor antagonists are necessary to clarify whether nap-related changes of mood can be prevented by anticholinergic blockade.