Critical interval hypotheses for depression.

Two chronobiologic models are presented for the etiology of 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 the light/dark cycle or photoperiod provides too little illumination during a critical photosensitive interval, which might in turn occur early due to a phase advance. Thus, depression might be treated by drugs or other manipulations which delay the phase of internal circadian rhythms. Depression might also be treated with bright illumination during the critical photosensitive interval. Preliminary experiments suggest that bright artificial light does have antidepressant effects. The optimal times for light exposure and the most responsive patient groups have not yet been identified.