The human body
How your body works and defends itself.
1:40Why do we sleep?
Sleep runs in cycles of about 90 minutes, with deep sleep early and REM later. Pressure from being awake and a 24-hour body clock set when you sleep.
2:02How vaccines train your immune system
A vaccine shows the immune system a harmless piece of a germ. The cell whose lock fits it wakes and divides, and some cells stay behind as memory.