The rhythmic variations of several hormones in man have been shown to be regulated by changes in the environmental light-dark cycle. In experiments with environmental lighting conditions the seasonal variation and the latitude of the study may influence the results. The nonvisual effect of light has recently been used for treating affective disorders. The clinical improvement of some depressed patients from the use of evening bright-light therapy may be related to an effect of light on the melatonin-rhythm generating system.