REM and Deep Sleep

The two most critical stages serve different masters:

• Deep sleep dominates the first half of the night — your body prioritizes physical restoration
• REM sleep dominates the second half — when your brain consolidates memories and processes emotions
• REM loss impairs learning, emotional regulation, and creative problem-solving

This is why cutting sleep short hurts so much. Sleep only five or six hours and you lose a large share of your REM, because it is packed into the later cycles.