How sleep works: what pulls you to bed, and what your brain does all night

Tonight, in a dream, you may run as fast as you can while your legs do not move at all: your brain holds them still on purpose. A sleeping brain stays busy all night. See what makes you sleepy (adenosine and your body clock), which hour of an all-nighter is the hardest and why, how a night moves between light sleep, deep sleep and REM sleep, and what coffee really does to your sleepiness.