What is known about dreaming
A dream is a sequence of images, thoughts and feelings experienced during sleep, usually with the sense that it is really happening. Almost everyone dreams, even people who rarely remember it. Dreams happen at several stages of sleep, but the most vivid and story-like ones are strongly associated with REM sleep, short for rapid eye movement. During REM sleep, brain activity looks surprisingly similar to waking activity, the eyes dart under the lids, and most voluntary muscles are temporarily kept from moving, which normally stops us from acting out dreams.
People also report dreams from non-REM sleep, though these tend to be shorter, plainer and more thought-like. Researchers still study how dreaming relates to the different stages and the brain activity in each.
What science can say with confidence is about how dreaming happens in the brain and when. Why we dream is much less certain. There are several theories, each with some support, and none has won out.
Why it is hard to study
Dreams are private. Researchers rely on people’s reports, usually given after waking, and memory of a dream fades quickly and can be distorted. Brain scans show what the brain is doing during sleep, but not exactly what a person is experiencing. And since dreaming is bundled together with sleep, it is hard to tell whether the dream itself does anything, or whether it is a by-product of other things going on during the night.
Psychoanalytic ideas
Sigmund Freud’s book on dreams, published around 1900, proposed that dreams are disguised expressions of hidden wishes, and that interpreting their symbols can reveal unconscious thoughts. The idea was hugely influential in popular culture. Most modern neuroscientists do not accept his specific claims, which are difficult to test. Still, the broader notion that dreams can reflect a person’s concerns and emotions has some support from studies of dream content.
Activation-synthesis
In the late 1970s, researchers J. Allan Hobson and Robert McCarley proposed that dreams arise when the brain, during REM sleep, produces random signals from the brainstem, and the higher brain tries to build a story out of them. On this view, dreams are not coded messages but the brain’s attempt to make sense of noisy activity. Later versions of the idea were revised, since dreams are not purely random, and they tend to reflect a person’s own life and concerns.
Memory consolidation
Another family of ideas suggests that sleep and dreaming help sort and store what we have learned. Studies show that sleep generally helps memory formation, and some research suggests REM and non-REM stages may play different roles. Whether dreaming itself, and not just sleep, contributes is unclear. Dreams often blend recent experiences with older memories, which fits the idea, but it does not prove it. The wider question of whether sleep is needed at all is covered in the article on the need for sleep.
Emotional processing
Some researchers suggest that dreaming helps process emotions, perhaps by replaying emotional experiences in a calmer chemical state, since certain stress-related chemicals are reduced during REM sleep. Dreams do often contain strong feelings, and people under stress commonly report more disturbing dreams. Evidence that dreams actually reduce emotional distress is mixed, and this remains an active research topic. It connects to broader topics such as human emotions and the stress response.
Threat simulation and rehearsal
A more evolutionary idea is that dreams evolved as a kind of practice for dealing with danger. This could explain why chases, falls and conflicts are common dream themes. The theory is hard to test directly, and critics point out that many dreams are mundane or do not match this pattern. Related notions propose that dreams let the mind rehearse social situations or try out new combinations of ideas, which might help creativity. These are interesting hypotheses, not established facts.
Dreams as a by-product
Some scientists think dreaming might serve no function of its own. According to this view, it is simply what consciousness looks like when a sleeping brain generates activity, much as a car engine makes noise without the noise being its purpose. It is hard to rule out, just as the other theories are hard to prove.
Common questions
- Do dreams predict the future? There is no scientific evidence that they do. Dreams draw on memories, worries and imagination, so they can seem to match later events by coincidence.
- Why do dreams feel strange? During REM sleep, regions involved in emotion and imagery are very active, while areas involved in logic and self-monitoring are less so. This may explain why odd events seem normal while dreaming.
- What about lucid dreaming? Some people become aware they are dreaming, and a few can steer the dream. This has been documented in sleep laboratories, and some people can learn it, but it varies a lot between individuals.
- Do animals dream? Many mammals and birds show REM-like sleep, but we cannot ask them what they experience, so it is hard to be sure.
How to think about it
It is reasonable to hold several of these ideas at once, as they are not mutually exclusive. Dreaming may serve more than one purpose, or may be a side effect of other processes, and both could be partly true. For a curious reader, the honest summary is that we know a good deal about the brain during dreaming and rather little about its purpose.
This article is general educational information and not medical advice. If sleep problems, nightmares or distressing dreams are affecting daily life, individual cases differ, and a qualified health professional can help.