Thats the fundamental problem with dementia, we have no idea really of what they take in and what they don't - and believe me, this can change on an hourly basis, a daily basis. I do believe at mid-dementia there is a correlation with sleep, or rather the lack of it.
The second one I really pine for is stroke victims - imagine losing, or having limited, your physical /communication capabilities to the point that folk consider you as "ijiuts" -when the cognitive brain is working perfectly well....a living prison that must be.