Stages
Deep, REM, and light — where recovery actually happens.
Where recovery happens
Sleep isn't one thing. Deep sleep repairs muscle, REM consolidates memory, and light sleep is the bridge between. Pulse reads all four stages.
The stages
deep muscle + tissue repair rem learning + mood light the long middle awake the interruptions
Why it matters
Total hours lie. Two people can both sleep 8 hours and recover completely differently. Stages are the truth.