We just learned about the Stage 3 - Non REM-Sleep.
Another part of sleep is Polysomnography.
This is a kind of sleep study where a doctor hooks someone up to machines to test how their brain, muscles and heart are working while they sleep.
![polysomnography](https://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Polysomnography_tester.jpg/545px-Polysomnography_tester.jpg)
(from: wikipedia - polysomnography)
Kid Facts - Blast from the past: Malleus