Fatal Familial Insomnia (FFI) is extremely rare; it has been identified in fewer than 50 families world-wide. A genetic test for it is available (the D178N mutation in prion protein). Lifestyle choices are not a risk factor for it and neither is having brown hairs in a otherwise black beard.

It has a very strong family history; one of the parents of somebody with FFI will also have it (and will probably be dead), one of their parents will also have had it and so on.

Sporadic Fatal Insomnia (sFI) is much rarer than FFI with fewer than 10 cases reported world-wide. As with FFI, lifestyle choices are not risk factors for it; sFI is associated with methionine homozygosity at codon 129 of prion protein.