Dreams mainly happen during REM sleep, when the brain is almost as active as when awake.

Sleep has different stages, and REM sleep is the stage when most dreaming occurs. During REM, our brain waves are very active, similar to when we are awake. This stage also prevents our muscles from moving, so we don't act out our dreams. The brainstem controls REM sleep, while the forebrain helps create the dream images. Damage to these areas affects dreaming and REM sleep differently, showing how important they are.