The circadian system is a homeostatic system that ensures appropriate co-ordination of nearly all the body's physiological and behavioural activities. In humans, one of the most well recognised activities that displays circadian rhythmicity is the sleep-wake cycle. Under normally entrained conditions, humans are designed to be awake during the day and asleep at night. This ensures optimal cognitive, endocrine, mood and
immune functioning, and high sleep quality and consolidation. Disruption to the circadian system produces disruption to the physiological and behavioural systems under its control, resulting in deficits in many aspects including cognition, mood, metabolic function and sleep. Circadian disruption occurs in shiftworkers and overseas travelers, and is also common in a range of circadian, medical and psychiatric disorders. In addition, sleep disturbance is typically comorbid with circadian disruption, which can then have further negative effects on a range of health and cognitive outcomes.
Changes in the circadian timing system and in sleep-wake behaviour are well documented in many psychiatric disorders, including bipolar disorder and depression. It is often reported that changes in these systems precede initial presentation of depressive or manic symptoms, as well as preceding relapse in patients with stable symptoms. This suggests that changes in sleep-wake timing and quality may potentially
be used as a biomarker of symptom stability and onset of some disorders, which would allow for early intervention to better manage these psychiatric disorders. In addition, targeting the sleep-wake and circadian systems concurrently with the treatment of the depression or bipolar disorder may provide an enhanced method of symptom management.