Are biological rhythms disturbed in depression?

In a large-scale investigation of circadian rhythms in endogenous depression, no free-running rhythms or indications of phase-advance were found in the patients compared with themselves after clinical recovery and with healthy controls. They exhibited a reduction of the amplitude of depression scales, partially due to a "ceiling effect" of highly elevated scores. A reduction of the amplitude of body temperature was probably related to a negative masking of the temperature rhythm by e.g. the patients' sleep disturbances. An increase in the amplitude of the cortisol rhythm, due to an elevation of the circadian maximum, was particularly pronounced in patients with significant diurnal variation in the severity of depression. It was thus probably related to a stress-induced positive masking of that rhythm. The acrophases of depression scores and the cortisol rhythm coincided roughly during but not outside depression. This may indicate a circadian modulation of the disease process and the activity of the hypothalamo-pituitary-adrenocortical system by the same clock-like mechanism within the hypothalamus.