Background: The human biological clock (circadian pacemaker) has long been thought to play a role in non-seasonal depression. A connection is suggested by the demonstration of 24-hour rhythms in mood, subjective and objective changes in sleep with depression, and reports of changes in the timing and amplitude of biological rhythms in depression. Furthermore, it is known that the neurotransmitter serotonin has a significant role in regulating biological rhythms and that drugs that act on serotonin (such as some antidepressants) are able to reset the biological clock in animals.