Hello - I am a LBD with Parkinsonism patient. I can give a little idea of what I am dealing with with sleep. This is only my experience.
Many folks with Parkinsonism / Dementia will experience sleep problems. For me - thrashing around, acting out dreams and weird sleep patterns.
REM sleep disorder is very common and effects the sleep. When you dont get REM sleep that can wear you out and your body wants it.
Have you ever considered a sleep study?

My body works overtime with the Parkinsonism which makes me tired physically. The mental part wears me out emotionally.
Sometimes I sleep because I dont want to face what is coming that day so I’d rather sleep.

Excessive daytime tiredness is also common. Sounds like that isn’t an issue with your husband. I had a problem with that for a while where I would be watching tv or looking at computer and start staring and out of it. IF there is someone with EDT and they arent getting that rest - then you hav to get the rest somehow.

I also have bouts with bad depression and that definitely makes me sleep a lot more. Depression / anxiety are both common with these conditions.

I wanted to chime in to give a little perspective from “in here”.

IF you or anyone else might have questions for someone in the “mid” stages of this I will be glad to answer as I can.

Peace
Larry H