Why Do We Dream? Sleep Quiz
The stages of sleep
Sleep isn't one long, even state. Through the night your brain moves through a cycle of stages: light sleep, then deep sleep, then a lively stage called REM sleep, short for rapid eye movement. One full cycle takes about 90 minutes.
Adults go through four to six of these cycles a night. Deep sleep comes mostly early in the night, while the REM periods get longer toward morning.
Most adults need about seven to nine hours of sleep. Teenagers need more, around eight to ten, and their body clocks naturally shift later, which is why early mornings can feel so hard.
What happens when we dream
We can dream in any stage, but the most vivid, story-like dreams happen during REM sleep. Brain scans show the brain is nearly as active then as when we're awake.
During REM sleep most of the body's muscles go limp, a kind of temporary paralysis. It stops us from acting out what we're dreaming. Our eyes, though, dart around under the eyelids, which gave the stage its name.
Everyone dreams several times a night, but most dreams fade within minutes of waking. Writing a dream down straight away is the best way to remember it.
Why we dream
Nobody knows for sure. One leading idea is that dreams are a side effect of the brain sorting and storing the day's memories and emotions. Another suggests dreaming lets us rehearse threats in a safe place.
A third idea, from the 1970s, says dreams are the brain's attempt to make a story out of random signals during REM sleep.
What scientists do know is that sleep itself is vital. It helps fix memories, and the brain clears away more waste while we sleep. Some people can even learn lucid dreaming: knowing they're dreaming while it's happening.