The human body has a "biological clock", a small region near the centre of the brain called the suprachiasmatic nucleus (SCN) that regulates sleeping, waking, and other body processes. During conditions of dim light, such as at nightfall, special photoreceptors in the eyes send signals to the SCN, which forwards the signal to the pineal gland, causing it to release melatonin into the bloodstream. Melatonin prepares the body for sleep by inducing drowsiness and lowering body temperature. When sufficient light returns, such as at dawn, the SCN signals the pineal gland to stop releasing melatonin and the body returns to a waking state.
But the rhythm of the internal clock can be disrupted due to shift work, jet lag, stress, delayed sleep phase syndrome, or other factors, leaving the body with insufficient melatonin at bedtime. Melatonin production also declines with age. Insufficient melatonin causes sleep disturbance.
Taking a melatonin supplement restores melatonin levels and improves sleep in at least two ways. First, melatonin produces a sedative effect by inhibiting the activity of the brain’s reticular activating system, which controls the wakeful state. Second, it temporarily resets the sleep phase of the biological clock, entraining it to a desired sleep time.