What happens during the different stages of sleep?

The answer

A typical night consists of 4 to 6 sleep cycles, each lasting roughly 90 minutes and progressing through four stages. Stage N1 is a light transitional phase lasting a few minutes. Stage N2 features sleep spindles and K-complexes, which are thought to help memory consolidation and protect against waking from external stimuli. Stage N3, or slow-wave sleep, is the deepest and most restorative phase, when growth hormone is released, tissues are repaired, and the glymphatic system clears metabolic waste (including amyloid beta) from the brain. REM sleep, characterized by rapid eye movements, vivid dreaming, and voluntary muscle paralysis, involves brain activity levels comparable to wakefulness and is critical for emotional processing and procedural memory. The ratio shifts throughout the night, with more deep sleep early and more REM later.