The circadian system and the therapeutics of the affective disorders.

Establishing that a circadian rhythm is abnormal tells us little about the cause, which can arise from changes in the patient's lifestyle, irregularities of the body clock or a malfunction in the process of entrainment of the clock. In a clinical context, such a range of possible explanations implies differences in the most appropriate mode of treatment. Against this background, the conventional view that the underlying abnormality in endogenous depression is due to a disorder of the body clock is challenged. The challenge is based on difficulties of interpretation of the clinical data and the results of studies on circadian rhythms in patients. It is suggested that the state of the circadian system in depression resembles its state in healthy individuals after time-zone transitions or in shift work maladaptation syndrome and that this disturbance should be seen as resulting from changes in the phasing of external zeitgebers rather than from an abnormality in the clock itself.