The Science Behind Why We Sleep

The Science Behind Why We Sleep Human beings, like all species on Earth, evolved to survive and thrive on a planet with a 24-hour cycle of day and night. According to some theories of sleep,   Trusted Source National Library of Medicine, Biotech Information The National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information. sleeping in one consolidated block at night allowed early humans to simultaneously avoid predators, conserve energy, and meet their need for rest. It also kept them from having to adapt to life in two very different conditions—daylight and darkness.   The biological patterns that help humans live according to the 24-hour day-night cycle are called circadian rhythms. These rhythms work alongside the sleep drive—a desire to sleep that grows in intensity the longer a person has been awake—to cause people to feel sleepy at night and alert in the morning.    Circadian rhythms, including the sleep-wake cycle, operate according to environmental cues. Every evening, as darkness sets in, the body begins releasing the sleep hormone melatonin—and every morning, with the arrival of light, the body’s melatonin levels become undetectable. An evening drop and morning rise in body temperature accompanies this cycle, enhancing sleepiness and alertness at the right times.