Why do we dream? What science knows about dreams

Why we dream is still an open question in science. Dreams are the stories and images the brain produces during sleep, most vividly in REM sleep, when it is nearly as active as when awake. The leading theories say dreaming helps sort memories, takes the sting out of emotional experiences, or rehearses threats, while another holds that dreams are a by-product of a busy sleeping brain. The evidence partly supports several of them.

In short

• Most vivid dreaming happens in REM sleep, but people also dream in other stages of sleep, often in simpler, more thought-like ways.
• The main theories link dreams to memory, emotional processing and threat rehearsal; none has been proven, and they may each explain part of it.
• Most dreams are forgotten within minutes of waking, partly because the brain chemistry of REM sleep is poor at laying down new memories.

When do we dream?

The modern science of dreams began in 1953, when Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago noticed that sleepers' eyes darted under their lids in regular bursts through the night. People woken during those bursts usually reported vivid dreams; people woken at other times mostly didn't. The bursts marked rapid eye movement, or REM, sleep, which arrives about 90 minutes after falling asleep and returns roughly every 90 minutes, with the longest REM periods toward morning.

During REM, according to the National Institute of Neurological Disorders and Stroke, the thalamus sends the cortex images, sounds and other sensations that fill dreams, while the brainstem relaxes the muscles of the limbs so that people don't act them out. The amygdala, a region involved in emotion, becomes increasingly active, one reason REM dreams are so often emotional.

For decades dreaming was equated with REM, but that turned out to be too simple. In a 2017 study in Nature Neuroscience, Francesca Siclari, Giulio Tononi and colleagues woke 46 volunteers more than 1,100 times while recording their brain waves in detail. People reported dreams from non-REM sleep too, and earlier studies had found dream reports on up to 70% of non-REM awakenings. Whether someone was dreaming showed up as a local change in a "hot zone" at the back of the brain, in both kinds of sleep, which let the team predict in real time whether a person woken from non-REM sleep would report a dream.

What are the main theories of why we dream?

No theory of dreaming has won; each explains some features of dreams and struggles with others. They are easier to compare side by side.

Theory	Who and when	The claim	Points for	Points against
Wish fulfillment	Sigmund Freud, The Interpretation of Dreams, 1899	Dreams disguise hidden wishes, to protect sleep	Dreams often draw on desires and worries	Hard to test, since any dream can be read as disguised; little modern support
Activation-synthesis	J. Allan Hobson and Robert McCarley, 1977	The brainstem fires semi-random signals in REM, and the cortex weaves them into a story	Explains bizarre, jumbled plots	Dreams are often coherent and tied to waking concerns, and also occur outside REM
Memory processing	Many sleep researchers, from the 1990s on	Dreams reflect the brain replaying and reorganizing new memories	Dreams mix recent events with old ones; dreaming about a task has been linked to doing better at it later	Most dreams aren't replays, and memory consolidation may happen without dreaming
Emotional regulation	Els van der Helm and Matthew Walker, 2009, among others	REM sleep keeps the content of emotional memories but softens their charge	REM is emotionally intense, and its brain chemistry fits the idea	Better supported for REM sleep itself than for the dreams
Threat simulation	Antti Revonsuo, 2000	Dreams rehearse dangers, so we respond better when awake	Threats and chases are common dream themes, across cultures	Many dreams are mundane, and the benefit is hard to measure

These theories need not exclude each other. Dreams could be the experience of the brain doing memory and emotional work, with some of their strangeness coming from the way the sleeping brain runs.

How could dreams help with emotions?

The idea with the most experimental work behind it focuses on REM sleep. In their 2009 review "Overnight therapy?" in Psychological Bulletin, van der Helm and Walker pointed out that REM sleep is unusual chemically: noradrenaline, a messenger tied to stress and alertness, is almost switched off, while the brain's emotional and memory regions are highly active. They proposed that REM lets the brain revisit an upsetting memory without the stress chemistry that came with it, keeping the facts while the emotional sting fades: the memory is kept, and the feeling is let go.

The same model offers an explanation for some nightmares. If the process fails, as may happen in post-traumatic stress disorder, the emotional charge stays attached and the memory can return night after night. That is a hypothesis about how sleep works, not a guide to interpreting any particular dream.

Why do we forget most dreams?

Most people dream every night, and REM sleep alone usually adds up to well over an hour, yet many adults remember only a few dreams a week, or none. Several things work against recall. The same low noradrenaline that may soften emotions also makes it hard to lay down new memories, so a dream isn't stored unless you wake during or just after it. Dreams are also fragile once you are awake: moving, checking a phone or thinking about the day overwrites them within minutes.

A useful way to see it is the morning sequence. If an alarm wakes someone out of a long late-night REM period, they're likely to wake mid-dream and remember it, at least for a moment. If they wake naturally from light non-REM sleep, the last dream may have ended many minutes earlier and left nothing. Lying still with eyes closed, and writing a dream down at once, are the usual ways people improve their recall.

What are nightmares and lucid dreams?

Nightmares are dreams frightening enough to wake the dreamer. Occasional nightmares are normal. Frequent nightmares that cause distress are a recognized condition, nightmare disorder, which the American Academy of Sleep Medicine says affects about 4% of adults, alone or alongside conditions such as PTSD, and which can be treated. Anyone troubled by repeated nightmares can raise it with a doctor.

In a lucid dream, the dreamer knows they are dreaming, and sometimes can steer the dream. A 2016 meta-analysis of 50 years of studies, by David Saunders and colleagues in Consciousness and Cognition, estimated that about 55% of people have had at least one lucid dream and about 23% have one a month or more. Lucid dreams are also a scientific tool: since around 1980, lucid dreamers have signaled from inside dreams with agreed patterns of eye movements. In 2021, four independent labs in the United States, Germany, France and the Netherlands went further. They put simple questions, including math problems, to 36 people during REM sleep, and six of them answered correctly on 29 occasions, using their eyes or facial muscles while still asleep.

Still argued

Whether dreams themselves have a function, or are a side effect of what the sleeping brain is doing for other reasons, is the central open question. Some researchers think the brain's work on memory and emotion during sleep would happen whether or not we experienced it as dreams. Others argue that the content of dreams matters, pointing to studies where dreaming about a task predicted improvement at it. Popular dream dictionaries that assign meanings to symbols have no scientific support; modern psychology treats dreams as reflecting the dreamer's concerns, not as coded messages.

Dreams are one of the clearest windows into how the brain builds experience from the inside, with almost nothing coming in from the senses, which is why researchers in consciousness study them as closely as sleep scientists do.

Check yourself

• Without looking back: what did Aserinsky and Kleitman discover in 1953, and why did it change the study of dreams?
• How does the emotional regulation theory explain both why dreams are emotional and why some nightmares repeat?
• Why are dreams so easy to forget, and what helps people remember them?

Sources

• Brain basics: understanding sleep — National Institute of Neurological Disorders and Stroke, 2025
• The neural correlates of dreaming — Siclari et al., Nature Neuroscience, 2017
• Overnight therapy? The role of sleep in emotional brain processing — van der Helm and Walker, Psychological Bulletin, 2009
• Real-time dialogue between experimenters and dreamers during REM sleep — Konkoly et al., Current Biology, 2021
• Treating nightmare disorder in adults — American Academy of Sleep Medicine
• Dream — Wikipedia