What Happens During REM Sleep, When Most Dreaming Happens?
Dreams feel like a strange extra life, but they sit inside a regular pattern of sleep that scientists can measure.
Two phases of sleep
The US National Heart, Lung, and Blood Institute says that when you sleep you cycle through two phases: rapid eye movement (REM) and non-REM sleep. The cycle starts over every 80 to 100 minutes, usually four to six times a night, and you may wake briefly between cycles. Sleep studies use sensors that record eye movements and brain activity to classify the stages.
Non-REM sleep has three stages. The third is deep sleep, or slow-wave sleep, and you usually spend more time in it early in the night.
What REM sleep is like
During REM sleep, NHLBI says, your eyes twitch and your brain is active. Brain activity measured then is similar to your activity while awake. Dreaming usually happens in REM sleep. Your muscles normally become limp, which prevents you from acting out your dreams.
You usually get more REM sleep later in the night. NHLBI notes that you get less of it in colder temperatures, because in REM sleep the body does not regulate its temperature properly.
It changes with age
Newborns spend more time in REM sleep. Slow-wave sleep peaks in early childhood and drops sharply in the teenage years. It keeps decreasing through adulthood, and older people may not have any.
What is still open
Why we dream is not settled. A paper in Current Biology says the functional role of REM sleep in normal biology remains an open question and one of the more intriguing mysteries. This article describes what is measured, not a proven purpose for dreams.
Sources
- NHLBI, Stages of sleep: https://www.nhlbi.nih.gov/health/sleep/stages-of-sleep
- Current Biology, on the open question of REM sleep function: https://www.cell.com/current-biology/fulltext/S0960-9822(17)31329-5
Facts checked: October 10, 2026.