Researchers describe sleep as promoting memory consolidation, and say memory reactivation during sleep is a primary mechanism behind that benefit. Brain regions recruited during learning become reactivated during sleep after the learning. REM sleep is also important for learning and memory. A good night's sleep improves learning and problem-solving skills, and sleep helps you pay attention and concentrate. Not getting enough sleep makes it harder to think clearly, react quickly and learn. In toddlers, napping appears to be necessary for memory consolidation, executive attention and motor skill development. Memory reactivation is tightly coupled to the cardinal neural oscillations of non-REM sleep.

Scientists linked REM sleep to dreaming by watching eyes. Aserinsky and Kleitman concluded that rapid eye movements were an objective signal of dreaming. 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 that REM goes with dreaming. 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. When a person was reawakened while the eyes were still, not in REM, they could rarely remember a dream. Even people who say they never remember a dream will recall one if awakened during a REM period.

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. By the time children start school, one full sleep cycle lasts about 90 minutes, and a night holds four to five cycles.

Dreaming is most common and intense during REM sleep, but it can happen in any sleep stage. Dreams during REM sleep are typically more vivid, fantastical or bizarre. Dreams are involuntary, and they can be forgotten in the blink of an eye. On average, most people dream for around two hours each night. 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. 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

• reactivation: becoming active again
• consolidation: memory strengthening
• physiologist: body scientist
• paralyzed: unable to move
• hypothesis: testable idea
• oscillations: rhythmic waves