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All of us have an occasional night when we can't fall asleep, 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, Explorer 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.