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In areas with reduced activity, the brain restores its supply of adenosine triphosphate (ATP), the molecule used for short-term storage and transport of energy. The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. The most pronounced physiological changes in sleep occur in the brain. This disrupts the release of the hormone melatonin needed to regulate the sleep-cycle. Common sources of artificial light include outdoor lighting and the screens of electronic devices such as smartphones and televisions, which emit large amounts of blue light, a form of light typically associated with daytime.

The use of artificial light has substantially altered humanity's sleep patterns. Humans may suffer from various sleep disorders, including dyssomnias such as insomnia, hypersomnia, narcolepsy, and sleep apnea parasomnias such as sleepwalking and rapid eye movement sleep behavior disorder bruxism and circadian rhythm sleep disorders. Sleep is a highly conserved behavior across animal evolution, likely going back hundreds of millions of years. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. The internal circadian clock promotes sleep daily at night. During sleep, most of the body's systems are in an anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the endocrine and immune systems. Dreams are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual paralysis of the body. Sleep occurs in repeating periods, in which the body alternates between two distinct modes: REM sleep and non-REM sleep. While sleep is different from wakefulness in terms of the ability to react to stimuli, it still involves active brain patterns, making it more reactive than a coma or disorders of consciousness.

During sleep, there is a decrease in muscle activity, and interactions with the surrounding environment are significantly reduced. Sleep can be described as a state of reduced mental and physical activity, in which consciousness is altered, and sensory activity is inhibited to a certain extent. Sleeping Girl, Domenico Fetti, circa 1615