Dreams occur mostly during a stage of sleep when the eyes move rapidly.

The longer version

Rapid eye movement sleep is one of several recurring stages the brain passes through during the night, and it alternates with deeper slow-wave sleep in cycles. During this stage the cortex becomes highly active again, the eyes dart about under closed lids, and the muscles of the body are largely held still by signals from the brain stem.

Vivid, narrative dreams are reported most often when people are woken from it, though dreaming also happens in other stages. Memory consolidation appears to be one of its functions, with recent experiences being replayed and sorted while the body stays motionless. The stage occupies a larger share of sleep in infants than in adults.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.