What is a biological clock?
Circadian rhythms are considered biological clocks.
A circadian rhythm is a process that occurs within a 24-hour cycle. An example of a circadian rhythm is the release of melatonin. Your suprachiasmatic nucleus (SCN) in your hypothalamus controls the production of melatonin in the pineal gland. Melatonin is a hormone in the brain that controls sleep. The SCN will enable the production of more melatonin during the afternoon (sleep) and will decrease production of melatonin during the day (alert).