Disturbances of the circadian timing system are implicated in the pathogenesis of numerous clinical syndromes, including sleep and affective disorders. Abnormalities of circadian rhythms can now be directly measured in the clinical laboratory and potentially corrected. Sleep scheduling disorders are most commonly due to phase misalignments between the endogenous circadian pacemaker and the socioenvironmental schedule. Current research is increasing our understanding of the influence of bright light exposure on the circadian timing system and has begun to be used successfully in the management of these conditions. There is substantial evidence that abnormalities of the circadian timing system are associated with depression. However, the application of new biologic rhythm diagnostic techniques would be required to establish whether circadian dysfunction is involved in the pathogenesis of these conditions. We propose a new hypothesis that phototherapy for seasonal depression may act by increasing an abnormally low circadian amplitude in those patients, such as that reported in endogenously depressed patients. The powerful effect of light on the circadian system indicates that phototherapy may become an important tool in the management of disorders of circadian etiology.