Fatal Filial Insomnia: A genetic disorder that usually doesn't show until your 30s. Your brain simply loses the ability to fall asleep. After a couple days, it's nearly impossible to complete even the simplest of tasks, a few more days and you're unable to form coherent sentences and experiencing vivid hallucinations you can't discern from reality, little longer and you slip into a coma due to your brain simply not being able to handle the stress of constantly being awake. Death follows shortly after.
There's no cure, no treatment, and if you had children before symptoms manifested, each one has a 50% chance of having the gene. Also, next time you're up at 3am and can't sleep, you're bound to be reminded of this. Have fun!