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. Researchers describe sleep as promoting memory consolidation, and say memory reactivation during sleep is a primary mechanism behind that benefit. A good night's sleep improves learning and problem-solving skills. Sleep helps you pay attention and concentrate, and not getting enough sleep makes it harder to think clearly, react quickly and learn.

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. The credit for discovering REM sleep is argued: one scholar suggests we should still honor Aserinsky and Kleitman as the predominant discoverers. Even people who say they never remember a dream will recall one if awakened during a REM period. People who are blind tend to have more dream components linked to sound, taste and smell.

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 stage 3, and during REM sleep most muscles are paralyzed in a condition known as atonia. 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. During REM sleep brain activity rises to levels like being awake, and breathing and heartbeat speed up. By the time children start school, one full sleep cycle lasts about 90 minutes, like an adult's, 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, and dreams during REM sleep are typically more vivid, fantastical or bizarre. On average, most people dream for around two hours each night. Dreams are involuntary, and they can be forgotten in the blink of an eye. How well people remember their dreams differs from person to person and can change with age. 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
• reactivated: made active again
• hypothesis: testable idea
• paralyzed: unable to move
• consolidation: memory strengthening