I don't know if there is a biomarker for "sleepiness" but sleep disorders can clearly be identified.
As I've tried to point out - the quality of sleep and REM is an important factor. You are right - insomnia is a possible characteristic of pre Alzheimer's, but more broadly it is a characteristic of CNS illness and also illness that is diagonsed as psychiatric which may be rooted in organic dysyfunction.
It really is an important biomarker that needs a lot more study.