Partial sleep deprivation (PSD), keeping a subject awake from 2 AM to 9 PM produces an acute mood improvement in 60% of patients with major depression. We sought to characterize the timing, subcomponent mood, and motor activity changes of this response. Thirty-seven subjects with major depression were rated with the 6-item Hamilton Depression Scale (HAM-6) at 1 PM and completed the Profile of Mood States (POMS) every 2 hr on the day before and day of PSD. Locomotor activity was monitored continuously during the trial with an automated device. Bipolar I patients responded more frequently than other groups. Positive mood responders had greater improvement than nonresponders in POMS subscales of depression, tension, confusion, and anger. The mood improvement increased steadily during the day, peaked in late afternoon, and declined thereafter. Responders showed significantly higher levels of locomotor activity on the baseline pre-PSD day than did nonresponders. All subjects increased motor activity following sleep deprivation, however.