Depressed patients show a reduction of natural killer (NK) cell activity which may be associated with specific depressive symptoms. The present study demonstrated that sleep disturbance and retardation, but not other depressive symptoms, were negatively correlated with NK activity in 38 depressed patients. Specific behavioral changes in depression such as sleep disturbance and retardation were found to predict 16% of the variance of cytotoxicity levels in depression.

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These results are consistent with the reported efficacy of sleep deprivation for major depressive disorder. However, the premenstrual dysphoric disorder subjects improved after the recovery sleep rather than directly after partial sleep deprivation. That late-night sleep deprivation did not have greater benefit than did the hypothesized sham treatment, early-night sleep deprivation, also suggests that placebo effects cannot be ruled out.