The hypothalamus contains the suprachiasmatic nucleus, the body’s main circadian clock.
The suprachiasmatic nucleus, or SCN, is a small cluster of neurons in the anterior hypothalamus, positioned above the optic chiasm. It receives light-related information from specialized retinal cells and uses that information to coordinate daily rhythms in physiology and behaviour.
The SCN helps time sleep and wakefulness, hormone release, body temperature, and other circadian processes. It sends timing signals to systems including the pineal gland, whose melatonin output tends to rise during biological night. The clock can continue oscillating without changing light, but external light is its strongest daily synchronizer.
The hypothalamus is sometimes reduced to a hunger or temperature centre, but it has broader roles in endocrine control, autonomic regulation, motivated behaviour, and biological timing. The SCN is the principal central pacemaker, while clocks also exist in organs throughout the body.