Each night your brain cycles through four sleep stages — N1, N2, N3, and REM — in roughly 90-minute intervals. Most adults complete five to six cycles per night. Understanding the structure is the foundation of every other sleep decision you make.
N1 is the brief transition into sleep (1–5 min). N2 is light sleep, the largest share of the night (~50%). N3 is slow-wave deep sleep, when physical recovery and immune work happen. REM is when dreams occur and memory consolidation is most active.
Waking during N3 deep sleep produces severe sleep inertia — the 'hit by a truck' feeling that can last 30 minutes. Waking at the end of a cycle (in N1/N2) feels dramatically lighter. This is why two more hours of sleep can sometimes leave you feeling worse, not better.
Early cycles are dominated by N3 deep sleep; later cycles tilt heavily toward REM. Cutting sleep short truncates REM disproportionately, which is why short nights hit memory and mood harder than physical recovery.
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