How does melatonin help regulate circadian rhythms?

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Hormone Many important physiological functions of vertebrates are controlled by steroid hormones.

Melatonin is a hormone secreted by the pineal gland that serves as a critical regulator of the circadian rhythm, or sleep-wake cycle. Its production is directly governed by light exposure detected by the retina.1

When the retina detects light, it sends signals to the suprachiasmatic nucleus (SCN) in the hypothalamus, which inhibits melatonin production to promote wakefulness and alertness during the day. Conversely, in the absence of light, the pineal gland is stimulated to produce melatonin. This nighttime surge induces physiological changes that facilitate sleep, such as lowering body temperature and respiration rate.1

The SCN contains a high concentration of melatonin receptors, which is where the hormone mediates most of its effects on the body's internal clock. By fluctuating in response to light and dark, melatonin acts as a biological signal that helps the body synchronize its internal processes with the external environment.1