Have you discussed this with the doctor treating the dementia? Is your husband in the late stages of the disease? By "most of the time" do you mean 16 hours per day? 20?
My husband took a med to help him sleep at night, and another med to cope with excessive daytime drowsiness. This combination worked for years. But as he entered the final stage it was just natural to sleep more than 20 hours a day. He went on hospice at that point.
I don't know that those medicines would have any impact on his sleep. But at the same time, it's my understanding that neither of those are intended to be taken anywhere near that many years. Mom also sleeps a lot. Not sure why, but she's 93 so she's probably just tired.
My husband took a med to help him sleep at night, and another med to cope with excessive daytime drowsiness. This combination worked for years. But as he entered the final stage it was just natural to sleep more than 20 hours a day. He went on hospice at that point.