Diurnal variation of depressive symptoms.

Diurnal variation of depressive symptoms appears to be part of the core of depression. Yet, longitudinal investigation of an individual's pattern, regularity, relation to clinical state, and clinical improvement reveals little homogeneity. Morning lows, afternoon slump, evening worsening-all can occur during a single depressive episode. Mood variability, or the propensity to produce mood swings, appears to be the characteristic that most predicts capacity to respond to treatment. Laboratory studies have revealed that mood, like physiological variables such as core body temperature, is regulated by a circadian clock interacting with the sleep homeostat. Many depressed patients, particularly bipolar patients, show delayed sleep phase (late chronotype). Even small shifts in the timing and duration of sleep affect mood state (sleep deprivation and sleep phase advance have an antidepressant effect). The implications for treatment are to stabilize mood state by enhancing synchronization of the sleep-wake cycle with the biological clock (eg, with light therapy).