I had this fear after not being able to sleep after 40 hours of being awake.
I heard about this a few months ago on youtube; I watched a video called "Top 10 worse diseases of all time" with fatal familial insomnia being number one. I never worried about it initially because I realised it was hereditary and so I would of known about it already and sporadic fatal insomnia is incredibly rare - one of the rarest things to get - ever!
But then after not being able to sleep well, the thought had kept on creeping up at night. This only happened a few nights but then when I couldn't sleep after 24 hours of being awake, I really started to panic. And it was through panicking that kept me awake for a further 14 hours. I had taken Zopiclone which had sent me straight to sleep. But I still panicked for another 2 weeks which kept me awake all night every other night and I could only sleep on sleeping pills.
But as soon as I stopped worrying about it being sporadic fatal insomnia, and after realising it was due to my anxiety, I started sleeping back to normal. However, insomnia crept back up and my anxiety about not sleeping came back. For the past 6 weeks, I have battled with my anxiety about not sleeping.
What helps me is this: remembering that insomnia is one of the most common problems with people who suffer from depression and anxiety disorders; think of all of those with clinical depression who spend days unable to sleep - some relying on drugs and alcohol. The fact that you have signed up to an anxiety forum is evidence that your insomnia is caused by anxiety. And therefore it is so easy for you to get into vicious cycles of not being able to sleep - which makes you anxious - which then further keeps you awake - which then makes you more anxious and it repeats.
But as easy it is to get yourself into the cycle, it is just as easy to get back out of it. It then becomes a battle of maintaining your anxiety to prevent yourself getting into the vicious cycle. But so long as you know it is your anxiety that causes it, and you don't worry about some other serious condition, you will find that it will start to become easy to control.
Insomnia is definitely not something that you can simply ignore but it helps to realise that it is a common problem. You are far from alone. There are loads of sufferers out there. And there is also a lot of help. I brought a self-help book on overcoming insomnia. I read a little but my sleeping routine is all over the place at the moment so it is hard for me to follow it right now.
Don't worry about it being sporadic fatal insomnia - concentrate on controlling your anxiety to help you sleep at night. This is easier said than done I know. But it is extremely rare that you have it. There are 7 billion people and there is only 9 known cases. Hang in there.