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Kulreet Chaudhary, MD, Neurology, answeredYou have an internal clock lodged deep within the brain that regulates your sleep -- the pineal gland. The pineal gland receives information about the sun through your eyes via the optic nerve. As the sun sets, the pineal gland is able to sense the change in light transmitted through your eyes and it begins to secrete a hormone, melatonin, to prepare your body for sleep. Exposure to bright light prevents the secretion of melatonin and darkness promotes it. Typically, within one to two hours after the sunset, you will begin to feel drowsy as the melatonin levels rise. This is the body's signal to go to sleep. By midnight, your melatonin levels have peaked and there is a gradual decline in melatonin levels after midnight.