Why do we dream? What sleep science does and doesn't know

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Everyone dreams, including people who swear they never do. Yet for something that fills a sizeable slice of every night, dreaming remains one of the least settled questions in biology. Researchers know a good deal about when dreams happen and what the brain is doing at the time. Why we dream at all is still debated. This guide separates the solid ground from the open questions.

What happens during a night's sleep

Sleep is not one long, even state. It runs in cycles, each typically lasting around 90 minutes, and an adult usually passes through several of them in a night.

Non-REM sleep comes first in each cycle. It moves from light sleep into deep, slow-wave sleep, when the brain's electrical activity becomes slow and rhythmic and you are hardest to wake.
REM sleep, short for rapid eye movement, follows. The eyes dart about under closed lids, brain activity looks surprisingly similar to being awake, and most of the body's muscles are temporarily switched off.
The balance shifts across the night. Deep sleep is concentrated in the early cycles, while REM periods grow longer towards morning. That is one reason you are more likely to wake from a vivid dream near dawn.

Where dreams fit in

REM sleep was described in the 1950s, and people woken from it very often report vivid, story-like dreams. For a while dreaming and REM were treated as the same thing. They are not quite.

Dreams also occur in non-REM sleep, though they tend to be shorter, vaguer and more thought-like.
The muscle switch-off during REM is thought to stop us acting out our dreams. When that system fails, as in a condition called REM sleep behaviour disorder, people can shout, kick or leap out of bed while dreaming.
We forget most dreams. Unless you wake during or just after one, the memory usually fades within minutes.

The main theories

There is no single accepted explanation for why we dream. The leading ideas are not mutually exclusive.

Memory processing. Sleep clearly matters for consolidating memories, turning the day's experiences into longer-lasting ones. Many researchers suspect dream content reflects this sorting process, which would explain why fragments of recent events turn up mixed with older memories.
Emotional regulation. One hypothesis holds that REM sleep helps take the sting out of difficult experiences, letting us remember an event while feeling it less sharply. It is an attractive idea but still a hypothesis.
Threat rehearsal. Another suggests that dreams let the brain practise responding to danger in a safe setting, which might explain why chases and falling are such common themes.
A by-product. Some scientists argue that dreams themselves have no purpose: the sleeping brain is busy with maintenance, and the mind stitches the resulting activity into a story.

What science does not know

It helps to be honest about the gaps.

Sleep has functions; dreams may or may not. Researchers can show that sleep, and REM sleep in particular, matters for learning and mood. Showing that the experience of dreaming itself does something is much harder.
Dream reports are unreliable data. They depend on what someone remembers on waking and how they describe it.
Dream dictionaries have no scientific basis. The idea that a given image carries a fixed hidden meaning is not supported by evidence, though dreams often do echo what is on your mind.

Nightmares and when to get advice

Occasional bad dreams are normal, especially during stress, illness or after a change in routine. Frequent nightmares that disturb your sleep, acting out dreams physically, or feeling exhausted despite a full night in bed are worth mentioning to a doctor, as they can sometimes point to a treatable sleep problem. This guide is general information rather than personal medical advice.

Getting better sleep, and better dreams

Keep a regular schedule, going to bed and getting up at similar times, weekends included.
Cut back on alcohol before bed. It can make you drowsy but tends to fragment sleep later in the night.
Keep the bedroom dark, quiet and cool, and give yourself time to wind down away from screens.
Keep a notebook by the bed if you want to remember dreams. Writing them down straight away is the most reliable way to hold on to them.

Have a strange recurring dream or a question about sleep research? Share it in the comments.