Biological rhythm disturbances in mood disorders.

From earliest times, psychiatrists have described biological rhythm disturbances as characteristic of mood disorders. The present flourishing of circadian biology has revealed the molecular basis of 24-h rhythmicity driven by 'clock' genes, as well as the importance of zeitgebers (synchronisers). Winter depression was first modelled on regulation of animal behaviour by seasonal changes in daylength, and led to application of light as the first successful chronobiological treatment in psychiatry. Light therapy has great promise for many other disorders (e.g. sleep-wake cycle disturbances in Alzheimer's dementia, bulimia, premenstrual disorder, depression during pregnancy) and, importantly, as an adjuvant to antidepressant medication in major non-seasonal depression. The pineal hormone melatonin is also a zeitgeber for the human circadian system, in addition to possessing direct sleep-promoting effects. Chronobiology has provided efficacious non-pharmaceutical treatments for mood disorders (such as sleep deprivation or light therapy) as well as novel approaches to new drugs (e.g. agomelatine).