Dreams

What dreams are, when they happen, why they're so strange, theories from Freud to activation-synthesis and threat simulation, lucid dreaming, and what brain scans of dreamers reveal.

When we dream

The discovery of REM sleep in 1953 suggested dreams belong to REM. People woken from REM report dreams most of the time — often long, vivid, bizarre and story-like. But people woken from non-REM sleep also report dreams fairly often, typically shorter and more thought-like. Most people have several dreams a night but remember few, especially if they don't wake during or just after them.

The dreaming brain

During REM sleep, brain imaging shows high activity in visual association areas, the amygdala and other emotional regions, and motor planning areas, with reduced activity in parts of the dorsolateral prefrontal cortex. That pattern fits the content of dreams: vivid imagery, strong emotions, movement — and a lack of critical self-reflection, which is why we accept bizarre events without question. A 2017 study by Francesca Siclari, Giulio Tononi and colleagues found that dreaming, in both REM and non-REM sleep, was associated with reduced slow-wave activity in a "posterior hot zone" at the back of the cortex.

Theories

Sigmund Freud's The Interpretation of Dreams (published in late 1899, dated 1900) saw dreams as disguised wish fulfilment; it was enormously influential but isn't supported by scientific evidence. In 1977 J. Allan Hobson and Robert McCarley proposed the activation-synthesis hypothesis: the brainstem generates bursts of activity during REM, and the cortex weaves a story from them. Others propose dreams help process emotions, consolidate and integrate memories, or simulate threats (Antti Revonsuo's theory) so we can rehearse responses safely. These ideas are not mutually exclusive, and none is proven.

Lucid dreaming

In a lucid dream, the dreamer knows they are dreaming and may control the dream. In the late 1970s and early 1980s, Keith Hearne in England and Stephen LaBerge at Stanford independently proved it scientifically: lucid dreamers made pre-agreed patterns of eye movements during REM sleep — signals visible on the recording — to show they were aware. A 2021 study even held two-way "conversations" with lucid dreamers, who answered simple maths questions with eye signals.