REM Sleep and Dreaming

During REM, the brainstem fires signals that resemble waking activity, so the cortex lights up in fast, irregular waves and the eyes move rapidly behind closed lids. At the same time, the brainstem sends a separate signal that blocks motor neurons in the spinal cord, so the muscles of the arms and legs go slack. This paralysis keeps the sleeper from physically acting out whatever is happening in the dream. The two systems run in parallel: one drives the vivid, waking-like brain activity, the other shuts down output to the muscles.

Where do the dreams themselves come from? Activation-synthesis theory says the brainstem generates essentially random bursts of neural activity during REM, and the cortex, which is built to find patterns and meaning, synthesizes that noise into a storyline. A dream about missing a bus isn't necessarily a disguised wish about some unresolved conflict; it can just as easily be the cortex stitching together random firing into something narratively coherent. This directly challenges the older Freudian view that every dream hides a symbolic message about waking life.

REM sleep also connects to the rest of the night's architecture and to learning. Deprive someone of REM by waking them each time they enter it, then let them sleep normally, and they show REM rebound: REM takes up a larger share of the following night's sleep, as if the body is making up a specific loss. REM sleep also supports memory consolidation, helping strengthen and organize information and skills learned while awake, linking this topic forward to how sleep loss affects learning and performance.