Where is the Clock?

The clock in humans is located in the suprachiasmatic nucleus(SCN), a distinct group of cells found within the hypothalamus. The SCN is only one part of the mechanism by which the "time" is kept. There are light receptors found in the retina which have a pathway, called the retinohypothalamic tract, leading to the SCN. The pineal gland is a pea-like structure found behind the hypothalamus in humans. The pineal gland receives information indirectly from the SCN[3]. It appears that the SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland, which secretes the hormone melatonin in response to this message. Nighttime causes melatonin secretion to rise, while daylight inhibits it[3]. Even when light cues are absent, melatonin is still released in a cyclic manner; yet if the SCN is destroyed, circadian rhythms disappear entirely[4].

The role of melatonin in humans is not clearly understood and is currently being investigated,but it is thought to play a role in photoperiodism in seasonally breeding mammals. The SCN is known to have hormone receptors for melatonin, so there may be a loop from the pineal back to the SCN. Researchers now use melatonin levels as an accurate marker of the circadian rhythm in humans.