In 1953, Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago discovered a stage of sleep marked by rapid eye movement, naming it REM sleep. During this stage the body's muscles are nearly paralyzed, while brain activity closely resembles the waking state.
A person cycles through REM sleep multiple times each night, with REM periods growing longer toward morning. Most vivid dreaming happens during this stage.
According to the Activation-Synthesis Theory proposed by Harvard's Allan Hobson and Robert McCarley, the brainstem fires off largely random neural signals during REM sleep. Rather than leaving them as noise, the cortex tries to 'synthesize' these scattered signals into a coherent story using existing memories and emotions — which is exactly why dreams often feel disjointed or shift scenes abruptly.
According to sleep researcher Matthew Walker's work, the brain revisits and reprocesses the day's emotional experiences during REM sleep. Because the stress hormone norepinephrine is suppressed at this time, emotional memories are replayed in something like a 'pain-free' state, gradually dulling their sharp intensity.
REM sleep is also closely tied to consolidating information learned during the day into long-term memory — one real reason a full night's sleep before an exam or big presentation genuinely helps.
Everyone dreams during REM sleep, but not everyone remembers it. A dream sticks in memory most clearly when you wake right at the transition out of REM sleep — which is part of why people who wake up more often during the night tend to recall more dreams.
If you want to remember dreams better, try lying still with your eyes closed for a moment right after waking, replaying the last scene before moving. Once you're fully awake and moving, most dreams fade within just a few minutes.