Seasonal affective disorders: the effects of light on human behaviour.

It has long been recognized that human moods are affected by seasonal changes: Hippocrates observed that 'of constitutions some are well or ill adapted to summer, others are well or ill adapted to winter'. It is only comparatively recently, however, that seasonal affective disorders (SAD) have been the subject of serious research. This indicates that mental depression--severe in a small minority of cases--may be related to low exposure to light in the winter months. In many cases phototherapy--which may influence melatonin production--brings relief.