The circadian rhythm describes the ability of the organism to synchronize physiological processes to a period of about 24 hours. The suprachiasmatic nucleus is the ‘switching cell’ of the circadian clock. It coordinates bodily functions such as body temperature, melatonin release (sleep hormone), blood pressure fluctuations and heart rate. A person’s sleep-wake rhythm is mainly disrupted by various light influences (e.g. less daylight time, exposure to artificial light in the evening and travelling across time zones).
There are two extreme types of the circadian rhythm, morning types ("lark") and evening types ("owl"). A lark’s peak performance hours are in early hours of the morning with sleep phases occurring between 20:00 to 5:00. An owl’s peak performance occurs late in the evening usually between 2:00 to 11:00.