You climb into bed, turn off the light, and close your eyes, but you can't fall asleep. For most of us, it's just one annoying night, but for people with fatal familial insomnia, or FFI, it's the beginning of the end. An extremely rare genetic disease passed down through generations, FFI's primary symptom is sleeplessness, but with a fatal twist: victims are dead within months. To understand FFI, National Geographic delves into the science of sleep the most elusive biological function we have to find out why we need sleep and what happens to US when we don't get it. Witness the pioneering research inside the sleeping brain as well as the mind forbidden to rest for days at a time.