If you look at the mechanism of glymphatic CNS clearance, the number of minutes it's active looks to be important - I was asking if there's something else going on to make these normally-short sleepers have normal clearance (e.g. proportionally more sleep spent in clearance mode, less need of clearance).

Given that the etiology of Alzheimer's is unclear, but short sleep (meaning less time for clearance) is a known risk factor. So if there's' an allele for normally-short sleep, that presents a chance to use mendelian randomization to see if there is a relation to dementia risk - or not.

Either way, we get useful data.