Dreams are experiences produced by the sleeping brain, and they can happen in both REM and non-REM sleep. They are often especially vivid during REM, when brain regions involved in imagery and emotion are active. Researchers do not yet know one definitive reason people dream; memory and emotion processing are active areas of study, not settled explanations.
Dreaming is not limited to one sleep stage
People often associate dreaming with rapid eye movement, or REM, sleep because vivid, story-like dreams are frequently reported after awakenings from REM. But dreams can also be reported after non-REM sleep. The NICHD notes that dreaming occurs in both, although REM dreams are generally more vivid. [1]
Sleep researchers study dreams by waking volunteers at different points in the night and asking what they experienced. Brain recordings and imaging add another window, but a dream is a private experience; scientists cannot watch its complete content directly. Reports can also be incomplete because dream memories fade quickly after waking.
The brain builds an experience from changing activity
During sleep, the brain is not simply switched off. Different networks become more or less active across sleep stages. In REM, areas involved in visual imagery, movement and emotion can be active, while some systems used for deliberate reasoning behave differently than when awake. Research reviews describe dreaming as a mix of perception, memory fragments, thought and emotion rather than a literal recording of the day. [2]
This helps explain why a dream can feel vivid while its events are disjointed or physically impossible. The brain is generating an experience from internal signals without the same sensory input and reality-checking that guide waking perception.
The purpose is still an open question
Dreams may relate to memory, learning or emotional processing, but evidence does not establish one universal function. Some researchers investigate how sleep supports memory and mood; that does not mean every dream has a message or that interpreting a dream can diagnose a person. Studies of dream-related brain activity are improving, yet connecting that activity to subjective experience remains difficult. [3]
Does everyone dream? Most people report dreams, though many are not remembered. Can a dream be predicted from brain scans? Researchers can identify patterns associated with dreaming in controlled studies, but they cannot reliably decode a person’s full dream story. Read about another everyday brain effect in why yawning can spread between people.
What the evidence does—and does not—show
| What is supported | Important limit |
|---|---|
| Dream reports occur after both REM and non-REM sleep. | There is no single established explanation for why we dream, and a dream is not a diagnostic test. |
This summary draws on the references listed below. Follow the source links for the observations and their context.
Sources and evidence
- NICHD — What Happens During Sleep? ↗
States that dreams occur in REM and non-REM sleep and that their purpose remains uncertain.
- Nature Reviews Neuroscience / PMC — The New Science of Sleep: From Cells to Large-Scale Societies ↗
Reviews observed brain activity associated with dreaming and the limits of current explanations.
- PubMed — What Are the Neural Mechanisms and Physiological Functions of Dreams? ↗
Scientific review emphasizing that the function of dreaming remains unresolved.
Sources consulted: 29 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: New explainer separates brain activity observations from hypotheses about why dreaming occurs. Updated 30 October 2026: added an evidence summary and clarified the limits of the explanation; repaired text formatting.