Melatonin—a hormone secreted by the tiny pineal gland, which is located at the base of the brain—is released when the level of light in the environment begins to drop with the approach of sunset.9 The higher the concentration of melatonin in the bloodstream, the sleepier the individual becomes. Conversely, when the sun rises and the environment becomes flooded with light, the production of melatonin by the pineal gland drops off, and the melatonin remaining in the blood is removed from the body by the kidneys. When the emerging humans began to use fire to ward off predators after sunset, they
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