When sleep duration and mood were monitored longitudinally in a 59-year-old woman with bipolar illness, sleep loss appeared to mediate the triggering of mania by psychosocial and pharmacological precipitating factors. This interpretation was supported by observations that mania could repeatedly be induced experimentally by depriving her of sleep for one night. The patient's data illustrate how sleep-loss might be a preventable cause of mania in a variety of situations.