Mum has vascular dementia. I have stopped her sleeping during the day by keeping her interested and busy but at night she sleeps in 2-3 hour sessions. This isn't especially healthy because it is hard to fall into deep REM(?) sleep. I just follow mum's body clock, go to bed with her, get up with her through the night and in the morning. If we are in bed(rooms) for 8-10 hours, we both usually get 5-8 hours sleep.