[Circadian rhythms and depression]

The degree to which nature's cycles influence our body is now obvious, owing to the recent progress of chronobiology. It seems thus that the harmonious internal organization of our circadian rhythms depends on the dual influence of the internal clocks as well as the external temporal informations. These endogenous pacemakers, able to spontaneously generate a rhythmicity the period of which is close to 24 hours, could be triggered by environmental informations, such as light/dark or sleep/Wake cycles, on a 24 hours schedule. The disruption of this entrainment could be involved in major affective disorders. This internal or external desynchronisation seems thus to be the central concept of different models for depression. The internal coincidence model suggests that a phase advance of the strong oscillator in reference to the weak oscillator causes depression. An external coincidence model suggests that depression is caused when light/dark cycle or photoperiod provide too little illumination during a critical photosensitive interval which could be located in the second half of the night. At last, a model comparing the biological rhythms of depressed patients and of healthy volunteers during temporal isolation, suggests that the internal clocks of depressed subjects could be temporarily "blind" to environmental time cues involving internal desynchronisation. These emerging findings should open a new therapeutic approach for depression.