I am tired of the term 'sundowners' and suggestions of lighting and scheduling. Habits are developed by learning, or appeal. They must find something appealing about staying up all night, in the early stages of dementia. What are the fears then of the daytime hours? My mother knows who the family is and the difference between night and day, although "I had no idea it was so late!" is a common statement. Yet she continues to sleep from dawn until dusk, after piddling around the kitchen. We have gone on trips, and managed to "reschedule" her routine fairly easily, for that period. She will go to bed at a reasonable hour and wake up with the rest of us. Upon coming home, however it isn't long before she reverts back to staying up all night. There has to be something other than a screwed up internal clock, or confusing night and day. Certainly someone can postulate as to "why" and not just tell this is part and parcel of dementia/Alzheimers, and leave it at that.
WHY because if they are up all night and we aren't, they can fool themselves into thinking they do not have the forgetfulness that is so apparent when they are awake during the day, there are no rules at night...our 87 year old did not like being alone, she did it for such a short time.
I'm not sure that eating at night is rewarding her at this point in her decline, as there is zero memory of anything, thus little chance for developing a new habit. She just responds to hunger pangs or as something to do, I imagine.
Instead of trying to figure out what's "wrong" with someone else, let's put ourselves in his/her shoes; and take it from there.
Mum goes to bed at midnight and gets up at noon which is causing huge problems for her diabetes as shes fasting too much and her mood swings are getting worse.