My mother had Alzheimers and slept a lot. We just let her sleep.
My husband has dementia and I just let him sleep whenever he wants to. I believe trying to process things when you have dementia is exhausting.

I think this disease is mentally and physically exhausting. I do think you should consult with your doctor to see if it’s possible something is wrong and is causing your loved one to sleep so much. My mom has a saying that when you sleep, you can’t make any trouble. From an old German lady ( my mom). Maybe someone here can provide you with more input. God bless you.