What happens in your brain each night while you sleep.
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Calculate my sleep cycles →The transition between wakefulness and sleep. Your muscles relax, heart rate slows, and brain waves begin to slow down. You can be easily awakened. This is when you sometimes feel that "falling" sensation (hypnic jerk).
Your body temperature drops, heart rate continues slowing, and the brain produces sleep spindles — bursts of activity that help consolidate memory. You spend about 50% of total sleep time in this stage.
The most restorative stage. Your body repairs tissues, strengthens the immune system, and releases growth hormone. It's very difficult to wake someone during deep sleep, and if you do, they'll experience severe sleep inertia.
Your brain becomes highly active — almost as active as when awake. This is when most dreams occur. REM is crucial for emotional processing, creativity, and long-term memory consolidation. Your muscles are temporarily paralyzed to prevent you from acting out your dreams.
One complete cycle (stages 1 → 2 → 3 → REM) lasts approximately 90 minutes. In a typical night of 7.5 hours, you go through 5 complete cycles. Early cycles have more deep sleep; later cycles have more REM. That's why the last hours of sleep are crucial for memory and learning.