Aserinsky and Kleitman concluded that rapid eye movements were an objective signal of dreaming. 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. The discovery of REM sleep by Aserinsky stimulated extensive research on the physiological side of REM sleep. Sleep comes in two types: non-REM sleep, which goes from light to deep, and REM sleep, when most dreaming happens.
William Dement, a physiologist who worked with Kleitman, later confirmed Aserinsky and Kleitman's hypothesis that REM goes with 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. How well people remember their dreams differs from person to person and can change with age.
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. Most REM sleep happens in the second half of a normal night's sleep. 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, and they can be forgotten in the blink of an eye.
During REM sleep most muscles are paralyzed in a condition known as atonia. Brain activity rises to levels like being awake, and breathing and heartbeat speed up. 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.
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. An adult's natural internal clock runs on average 24.2 hours, and daily light cues reset it to 24 hours. Teenagers' body clocks shift later, so they feel sleepy later at night.
Dolphins sleep with only half of their brain at a time, so they can stay aware of danger. 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. Sleeping with half the brain may help migratory birds make long flights, perhaps sleeping while gliding. Migrating birds sleep significantly less while migrating: white-crowned sparrows get only a third of the sleep they get when not migrating.
Researchers describe sleep as promoting memory consolidation, and say memory reactivation during sleep is a primary mechanism behind that benefit. Dreaming has been associated with the consolidation of memory. REM sleep is important for learning and memory. 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.
Words to know
- physiologist: body scientist
- hypothesis: testable idea
- paralyzed: unable to move
- adaptation: helpful inherited trait
- cyanobacterium: single-celled organism
