A subsequent study has not found any link between the ADRB1/DEC2/other gene mutations and various sleep conditions:

- https://pmc.ncbi.nlm.nih.gov/articles/PMC9499244/

I am incapable of sleeping more than about 5.5 hours (unless I'm severely sleep deprived). So to me it seems likely that FNSS (familial natural short sleep) is a thing, but more research is needed I guess.