We studied the effect of two doses (0.9 mg and 1.8 mg) of Hypericum perforatum (St John's wort) on the sleep polysomnogram of healthy subjects using a placebo-controlled, cross-over design. Both doses of hypericum significantly increased the latency to rapid eye movement (REM) sleep without producing any other effect on sleep architecture. Our data are consistent with the proposed clinical antidepressant efficacy of hypericum, and raise the possibility that its pharmacological mechanism of action may be similar to that of conventional antidepressant medication.