Sleep has stages, and brain waves show clear patterns for each one. 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. Heart rate is slowest then too. Muscles gradually relax through the stages of non-REM sleep, and the body's total energy use drops. Hormone levels change during the different sleep stages, and the quality of sleep may also affect hormone production in the day.
Most REM sleep happens in the second half of a normal night's sleep. 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. During REM sleep most muscles are paralyzed in a condition known as atonia. On average, most people dream for around two hours each night. Some people dream in color while others dream in black and white. Dreams are involuntary: a person does not choose them. Dreaming has been associated with the consolidation of memory, and REM sleep is important for learning and memory.
How did scientists first link REM to dreaming? 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. The discovery of REM sleep by Aserinsky stimulated extensive research on the physiological side of REM sleep. Even people who say they never remember a dream will recall one if awakened during a REM period. 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.
Researchers suggest that sleeping in one block at night let early humans avoid predators and save energy. Many kinds of animals, not only people, have REM periods during sleep. Scientists have watched elephants travel for nearly two days without sleeping at all. Migrating birds sleep significantly less while migrating: white-crowned sparrows get only a third of the sleep they get when not migrating. Sleeping with half the brain lets birds protect themselves from predators and may help migratory birds make long flights, perhaps sleeping while gliding. Elephants sleep very little: some researchers have recorded a total of just 2 hours a day.
The body builds a sleep drive, a desire to sleep that grows stronger the longer a person has been awake. An adult's natural internal clock runs on average 24.2 hours, and daily light cues reset it to 24 hours. Scientists found a protein in fruit flies that helps control normal daily biological rhythms. As of November 2026, the International Space Station orbits Earth every 90 minutes, so crew members see 16 sunrises each day. The frequent change from darkness to light makes it hard for the body to keep a natural circadian rhythm. As of November 2026, LED light assemblies let the crew adjust the color spectrum and intensity of the light.
As of November 2026, even the most basic question, why we dream at all, is still debated by scientists. A good night's sleep improves learning and problem-solving skills. Studies show that sleep deficiency changes activity in some parts of the brain. Not getting enough sleep makes it harder to think clearly, react quickly and learn. Sleep helps you pay attention and concentrate. How much sleep a person needs depends on age, lifestyle, health and sleep patterns, and it can differ from person to person. Good sleep is uninterrupted and refreshing. Sleep helps the body restore energy and save energy.
Words to know
- consolidation: memory strengthening
- predators: animals that hunt
- deficiency: not enough
- objective: measurable
- migrating: traveling each season
