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Mental
Dreaming and Sleep States
Sleep is a daily state of reduced activity and altered consciousness that cycles between REM and non-REM sleep, and dreams happen mostly during REM.
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The idea
Sleep is a state of reduced mental and physical activity in which consciousness is altered and some of the senses are dialed down. Muscles become much less active, and a sleeper interacts far less with the world around them. Even so, the brain stays busy, which is why a sleeping person reacts more than someone in a coma.
Sleep happens in repeating cycles that switch between two modes, rapid eye movement (REM) sleep and non-REM sleep. In adults, one cycle lasts about 90 minutes, and later cycles hold more REM sleep. During REM sleep the eyes dart around, the muscles go limp, and brain activity looks a lot like being awake. Dreams, which are strings of images, ideas, emotions and sensations, happen mainly during REM sleep, though people dream in non-REM sleep too. Humans generally spend more than two hours dreaming each night.
Sleep also follows a daily rhythm, because the body's internal circadian clock promotes sleep at night, when it is dark. Most people today sleep in one main period each day, which is called monophasic sleep, and a nap on top of night sleep is a common kind of biphasic sleep. Historical studies suggest that before industrialization, many societies commonly slept in two or more segments during the night. During sleep, the body also does repair work that helps restore the immune, nervous, skeletal and muscular systems and supports mood and memory.
Check yourself
Question 1 of 3
About how long does one sleep cycle last in adults?
Composition
what it's made of
- Non-REM sleep — The sleep stages other than REM, which make up the rest of each cycle.
- REM sleep — A phase with rapid eye movements, limp muscles, wake-like brain activity and vivid dreams.
- Dreams — Images, ideas, emotions and sensations that occur involuntarily in the mind during sleep.
- Sleep cycles — Repeating rounds of non-REM and REM sleep, each about 90 minutes long in adults.
- Circadian clock — The body's internal daily clock, which promotes sleep at night.
- Sleep patterns — How sleep is divided across a day: monophasic (one period), biphasic (two) or polyphasic (more than two).
Connection
how it relates
Under Human Consciousness
Change
how and why it evolves
- Past
- People have long tried to understand dreams, and the Babylonians practiced dream interpretation in the third millennium BCE. In 1953, Nathaniel Kleitman and his student Eugene Aserinsky defined rapid eye movement and linked it to dreams.
- Now
- Artificial light has greatly changed how people sleep. Screens on phones and televisions give off blue light, which disrupts the release of melatonin, the hormone that helps regulate the sleep cycle.
- Next
- Scientists still do not know exactly why we dream, and the purposes of sleep are the subject of much ongoing research. One idea being tested is that sleep helps the brain clear out waste.