REM (Rapid Eye Movement) sleep is a unique phase of the sleep cycle characterized by heightened brain activity, flickering eye movements, and temporary muscle paralysis. Often referred to as "active sleep," it is the stage where the most vivid and narratively complex dreaming occurs. During this time, your brain processes emotions, consolidates memories, and clears out unnecessary information, making it vital for mental health and cognitive function. On average, you enter REM about 90 minutes after falling asleep, with each subsequent cycle lengthening as the night progresses.