Why do we need sleep, and why do we dream?

What happens in the brain while we sleep?

NINDS describes two basic types of sleep, REM and non-REM. Non-REM sleep runs from light sleep, where we spend the most time, to the deep sleep we need to feel refreshed. REM sleep first arrives about 90 minutes after we fall asleep. The arm and leg muscles go limp, which keeps us from acting out our dreams, and brain waves look closer to waking.

In a 2017 study, people woken from both REM and non-REM sleep sometimes reported dreams, and in both, dreaming went with fewer slow brain waves toward the back of the brain. But in a small 2020 test, analysts who could not see the dream reports used brain-wave readings, including that signal, to spot dreaming and did no better than chance. For how brain activity becomes experience, see Is the mind just the brain? When REM’s muscle shutdown lingers after waking, a person can be awake but unable to move; see our answer on sleep paralysis.

What does sleep do for us?

It helps us keep what we learn. A 2021 meta-analysis (a study that pools many studies) combined 823 results from 177 papers published from 1967 to 2019. People recalled more after sleep than after an equal time awake. The benefit was moderate, and it shrank but held after a correction for results that may have gone unreported. A leading explanation, from a 2013 review, is that the sleeping brain replays new memories during deep sleep and moves them into long-term storage. In a 2020 meta-analysis of 91 experiments, playing sounds or smells tied to a lesson during non-REM sleep helped memory, but playing them in REM sleep or while awake did not. Still, a 2021 review by Maren Cordi and Björn Rasch reported that several key findings behind these theories could not be replicated or held only under some conditions, and that the effects seem smaller than once assumed.

It may help clean the brain. In a 2013 study of live mice, sleep came with a 60% increase in the space between brain cells, and a waste protein called beta-amyloid was cleared faster. But a 2024 study from another lab, also in mice, found clearance “markedly reduced, not increased” during sleep. A 2025 mouse study with the same senior author as the 2013 study, Maiken Nedergaard, tied this cleaning, which scientists call the glymphatic system, to slow pulses of blood vessels in non-REM sleep. In a 2021 study of 24 people in Norway who had a tracer put into their spinal fluid, one night without sleep slowed the tracer’s clearing from most brain regions. We label this cleaning role Supported, still debated.

Too little of it does real harm. In a 2003 lab study, healthy adults held to 6 hours or less in bed for two weeks slipped on attention and thinking tests as much as people kept awake for up to two nights, yet seemed largely unaware of it. Sleep experts link regularly sleeping under 7 hours to weight gain, diabetes, high blood pressure, heart disease, depression and accidents, though, as NINDS notes, it is not always clear which causes which.

Why do we dream?

Dreams are hard to study because, as a 2021 study notes, reports made after waking are open to “distortion and forgetting.” Several theories compete:

• Threat rehearsal. In 2000, Antti Revonsuo proposed that dreams evolved to rehearse threats so we could practice escaping them. A 2005 study by his team found that severely traumatized Kurdish children reported more dreams, with more threats that were also more severe, than less traumatized Kurdish children or Finnish children who had not faced trauma. But a 2008 study by another team found fewer threat dreams in a high-crime area of South Africa than in Wales, and realistic survival threats in under 20% of dreams.
• Memory processing. In a 2010 study of 99 people learning a virtual maze, the four nappers whose dream reports mentioned the maze improved about ten times as much as other nappers; thinking about it while awake did not predict any gain. The authors noted the small numbers and did not claim the dreams caused the gain. Erin Wamsley, who led it, saw the link again in a small overnight study in 2019, and her team’s 2023 meta-analysis of 16 studies tied task dreams to better memory, mainly for dreams from non-REM sleep. A link is not a cause.
• Activation-synthesis. In 1977, Hobson and McCarley proposed that during dreaming sleep the brainstem fires in bursts, and the forebrain builds a dream by comparing those signals with stored memories.
• The overfitted brain. In 2021, Erik Hoel of Tufts University borrowed an idea from artificial intelligence: a computer network that learns its examples too narrowly fails on new ones, and programmers often fix this by adding noise. He argued that dreams, with their odd, scrambled scenes, add that noise so the brain can generalize, and he listed ways to test the idea.

NINDS adds another idea: dreaming may help us process emotions. The strongest objection is that dreams may have no job at all. As both Revonsuo and Hoel note, several theories treat dreams as by-products of the sleeping brain. With tests pointing different ways, we label the purpose of dreaming an Open question.

Do dreams mean anything?

What research can say. In a 2003 diary study, people who spent more time on an activity by day tended to dream about it more, though focused tasks such as reading and writing rarely showed up. NINDS adds that people under stress or anxiety are more likely to have frightening dreams. In studies published in 2009, people from Eastern and Western cultures believed dreams reveal hidden truths, but found a dream more meaningful when it fit what they already believed, about friends or about God. They also reported being as reluctant to fly after dreaming of a plane crash as after learning of one.

What faith traditions teach. For many believers, some dreams matter because of where they come from. In the Hebrew Bible, God says of prophets, “I speak with them in a dream” (Numbers 12:6), and Joseph, meeting two prisoners troubled by their dreams, says, “Surely God can interpret!” (Genesis 40:8). In the Gospel of Matthew, the angel of the Lord appears to Joseph, the husband of Mary, in a dream (Matthew 1:20). The Quran tells of Joseph (Yusuf) seeing eleven stars, the sun and the moon bowing down to him in a dream (Quran 12:4), and in Sahih al-Bukhari the Prophet Muhammad is reported to say, “A true good dream is from Allah, and a bad dream is from Satan.” In a 2023 Pew survey, 46% of U.S. adults said a dead family member had visited them in a dream; Pew did not ask what they meant by it. See also Is it normal to feel that someone who died is still with me? Whether a dream was sent by God or a loved one is Beyond science.

Where people of faith and skeptics can agree

• Many dreams echo the day. Research finds waking life in dreams. The Talmud records a rabbi teaching that a person is shown in a dream only “the thoughts of his heart when he was awake” (Berakhot 55b), and a saying recorded in Sahih al-Bukhari names “the reflection of one’s thoughts and experiences” as one of three kinds of dreams.
• Not every dream is a message. Both of these texts tell kinds of dreams apart, and research shows we find dreams more meaningful when they fit what we already believe.

Why do we forget most of our dreams?

Dreaming and remembering a dream are different questions. One proposed explanation for forgetting is that a dream does not become a lasting memory.

In a 2017 laboratory reanalysis, 36 frequent and infrequent dream recallers slept while sounds were played. Frequent recallers had longer awakenings during sleep. The authors proposed that these intervals allow dream memories to be stored.

This association supports a memory-encoding account; it does not establish why every forgotten dream disappears. The sounds also make this different from an ordinary night.

What is a lucid dream, and can anyone learn to have one?

A lucid dream is one in which you recognize that you are dreaming while the dream continues. That awareness can occur without control over the story.

In a 2020 international study, selected adult volunteers reported more lucid dreams during a week of training than during their baseline week, including participants without prior technique experience.

That suggests some people can learn to have lucid dreams more often. It does not show that anyone can reliably do so: many volunteers dropped out, reports were self-recorded, and practice lasted only one week.

What do people often get wrong?

“I never dream.” NINDS says everyone dreams, but we may not remember most of our dreams.

“Dreams only happen in REM sleep.” They happen in non-REM sleep too.

“I can catch up on the weekend.” NINDS says extra weekend sleep may not make up for all you missed.

How can you sleep better and check this yourself?

• Know the target. The CDC recommends 8 to 10 hours for teens, 7 or more for adults 18 to 60, 7 to 9 for ages 61 to 64, and 7 to 8 from age 65.
• Build habits. The CDC and the American Academy of Sleep Medicine suggest the same bedtime and wake time every day, a quiet, cool bedroom, screens off at least 30 minutes before bed, and no caffeine in the afternoon or evening. If you can’t fall asleep after 20 minutes, the Academy suggests getting up for a quiet activity in dim light.
• Get help when needed. See a doctor if you have trouble sleeping or feel unusually tired during the day; NINDS says most sleep disorders can be treated effectively. For chronic insomnia, the Academy calls cognitive behavioral therapy the most effective long-term treatment.
• Keep a dream journal. Write down your dreams for a few weeks and ask: how often does your day show up at night?

The answer stays the same: we need sleep, its deepest purpose is unsettled, and why we dream remains an open question.