My 78 yr old husband sleeps well at night, but he no sooner gets up when he's in the chair sleeping, sometimes even before breakfast! He often sleeps after breakfast until nearly lunch, then after lunch until almost dinner. I try to get him out to do things, but he says he's too weak. He also has Parkinson's which isn't helping. I don't know what to do to help him.
Anyway, whatever type of dementia he has, the excessive daytime sleepiness may be treatable. My husband took Provigil for that condition and it was very helpful for years. Have you discussed this problem with your husband's doctor?
As my husband entered the end stage he began to sleep more and more, and the medication became ineffective. Eventually he sleep more than 20 hours a day.
My husband's doctor knew that his condition (Dementia with Lewy Bodies) could not be cured, but he felt strongly about giving his patients the best quality of life they could have. So he treated the excessive daytime sleepiness right up until the last stage. I am grateful for that.