The human body
How your body works and defends itself.
0:36Why do we sleep?
Sleep runs in 90-minute cycles that clean and repair the brain and file the day’s memories, timed by rising sleep pressure and a 24-hour body clock.
0:28How vaccines train your immune system
A vaccine lets your immune system meet a harmless copy of a germ, learn its shape and remember it, so the real germ meets a fast response.