Depression, mania and probably starvation all induce changes in pineal function. At present it is unknown what secondary effects on the endocrine and other systems are produced by these changes. Studies in rats have established an entraining effect of melatonin on locomotor activity and a feedback effect on the pineal itself. Studies of jet-lag and of sleep dysregulation in a blind subject established that melatonin treatment has a synchronizing effect in these conditions. Further investigations will be necessary to establish whether melatonin reduction in depression and other disorders leads secondarily to dysregulation of other circadian rhythms.