Any idea why mom sleeps so much?

ITs Called "Sleep Disorders"
Your Mom may sleep so much because when They are asleep they are not in a restful sleep. If she do not reach a state of REM sleep it is likely that when she wake up she will not feel well rested.

How long is she sleeping midday that you are concerned? My mom is 81 (no dementia) and she takes an afternoon nap from about 1-3. She goes to bed early, about 9pm and wakes up 5am.

My MIL had vascular dementia and when she was mid-stage the nursing staff worked to get her into the habit of taking an afternoon nap from 1-2:30 - she was sundowning and that nap really helped. She would go through periods of sleeping a great deal, morning and afternoon naps, and she would go through napless periods.

She is coming up on 85 years old this October. She will get tired again around 11 pm and I begin to get her ready for bed in that hour. She "seems" to be sleep most of the night, but I cannot tell because she does not get up if she is awake. She will wait until sunlight. She said that that's all she knows --night and day, so she may know enough to stay in bed while it's dark. She has not exhibited "sundowners syndrome" yet and does not unlock the door and walk out during the day either. she doesn't eat big meals, but she does eat well. She is currently not in her home, but at my house, but she did this at home ssitting in her diniring room chair while facing the TV as well.

How is she sleeping at night?
How old is she?
I am new at all of this stuff, my mom had a stroke within the last 6 months, she doesnt sleep well at night, but boy can she sleep during the day!