Researchers describe sleep as promoting memory consolidation, and say memory reactivation during sleep is a primary mechanism behind that benefit. In one study, hippocampal reactivation during sleep was selective for the spatial memories that recruited the hippocampus during learning. Hippocampal activity during sleep was higher than in control groups who did not perform a pre-sleep learning task. In another, stimulating the upstate of slow oscillations during sleep also enhanced picture recognition accuracy the following day. Patterns of neuron firing seen during learning are replayed during sleep in temporally compressed sequences, mainly in the hippocampus but also with learning-related cortical sites.

The number of replay events during sleep is correlated with how much memory improves from before to after sleep. Reactivation events occur most strongly during ripple-spindle-slow oscillation events, and memory reactivation is facilitated by an interplay of hippocampal ripples. Brain regions recruited during learning become reactivated during sleep after the learning, and memory reactivation is tightly coupled to the cardinal neural oscillations of non-REM sleep. REM sleep is also important for learning and memory, and not getting enough sleep makes it harder to think clearly, react quickly and learn.

Aserinsky and Kleitman concluded that rapid eye movements were an objective signal of dreaming, and a 1953 paper reported REM sleep. In their early study the first REM period came about an hour after sleep began and lasted from five to ten minutes. William Dement, a physiologist who worked with Kleitman, later confirmed their hypothesis. In 1957 Dement and Kleitman published a paper on cyclical variations in EEG during sleep and their relation to eye movements, body motility and dreaming. The credit for discovering REM sleep is argued: one scholar suggests we should still honor Aserinsky and Kleitman as the predominant discoverers.

People remember dreams in different ways. Even people who say they never remember a dream will recall one if awakened during a REM period, and when a person was reawakened while the eyes were still, not in REM, they could rarely remember a dream. How well people remember their dreams differs from person to person and can change with age. People who are blind tend to have more dream components linked to sound, taste and smell. Some people dream in color while others dream in black and white.

Brain waves measured during sleep show clear patterns for each sleep stage. In the early parts of non-REM sleep brain waves slow down a great deal, while in REM sleep brain activity speeds up and shows very different brain waves. Breathing slows during non-REM sleep and reaches its lowest rate in the deepest stage, stage 3, and heart rate is slowest then too. Muscles gradually relax through the stages of non-REM sleep, and the body's total energy use drops. Most REM sleep happens in the second half of a normal night's sleep, and during REM sleep most muscles are paralyzed in a condition known as atonia.

An adult's natural internal clock runs on average 24.2 hours, and daily light cues reset it to 24 hours. Even a single-celled organism, the cyanobacterium Synechococcus elongatus, has a fully functional circadian rhythm. Scientists found a protein in fruit flies that helps control normal daily biological rhythms. The human master clock is a group of nerve cells in the brain called the suprachiasmatic nucleus, and it runs on a cycle slightly longer than 24 hours.

Teenagers' body clocks shift later, so they feel sleepy later at night and want to wake up later in the morning. As kids get older they spend less time in REM sleep and their sleep cycles get longer, while babies spend more time in REM sleep and have shorter cycles. In toddlers, napping appears to be necessary for memory consolidation, executive attention and motor skill development.

Dolphins sleep with only half of their brain at a time, and keeping half the brain awake is likely an evolutionary adaptation that protects dolphins against sharks. Birds also sleep with one side of the brain asleep while the other stays awake. Migrating birds sleep significantly less while migrating: white-crowned sparrows get only a third of the sleep they get when not migrating. Scientists have watched elephants travel for nearly two days without sleeping at all.

Dreams are typically more vivid, fantastical or bizarre in REM sleep, they are involuntary, and they can be forgotten in the blink of an eye. As of November 2026, even the most basic question, why we dream at all, is still debated by scientists. One view is that dream content may be a form of distorted instant replay in which recent events are reviewed and analyzed, while another view holds that dreaming is just a by-product of sleep with no essential purpose or meaning. Dreaming has been associated with the consolidation of memory.

Words to know

• hippocampus: memory brain region
• consolidation: memory strengthening
• oscillations: rhythmic waves
• physiologist: body scientist
• circadian: daily body rhythm