Sleep, Explained
A guided route through how sleep actually works, from your body clock and sleep pressure to the deep and dream stages that restore body and mind.
14 steps · roughly 42 minutes to read · Reviewed by Shariq Refai, MD, MBA
Circadian Rhythm
The circadian rhythm is your internal daily clock, tuned mostly by light.
Chronotype
Your biology sets when you naturally feel alert and sleepy.
Light Exposure
When you get light, especially bright light, tunes your sleep-wake clock.
Melatonin
The brain's night signal that prepares the body for sleep.
Sleep Pressure
Sleep pressure is the drive to sleep that rises with every waking hour and clears when you sleep.
Two-Process Model of Sleep
Sleep timing comes from sleep pressure and the circadian clock working together.
Sleep Architecture
Sleep has a structure, cycling through stages all night long.
Slow-Wave Sleep
The deep sleep that restores the body and locks in certain memories.
REM Rebound
Missed REM sleep gets repaid with more REM on later nights.
Sleep Spindles
Short brain bursts during sleep that help store new learning.
Glymphatic System
A brain cleanup network that flushes waste, especially while you sleep.
Sleep Efficiency
A measure of how much of your time in bed is real sleep.
Wind-Down Routine
A consistent set of soothing steps that ease you toward sleep.
Sleep Debt
Sleep debt is the quiet tally of the rest you didn't get.