My 87-year-old mom may have vascular dementia and has heart issues. She seems to be sleeping more in her recliner at night.

My mom started sleeping in her recliner when she was having back pain. She was more comfortable in the recliner than flat on her back. I got her an adjustable bed so she could sleep in the "zero gravity" position.

Leave her where she is In the chair and do not wake her .

I have sleepless nights, and sometimes go to the recliner. So it could be just that she is waking during the night and falls back to sleep in the recliner.

Are you noticing any increase in edema or any shortness of breath when flat in bed? Does she complain of difficulty with breathing. I would discuss this with MD, esp if those things I mention are at all present. You might consider measuring her O2 oximetry when flat in bed and when sitting. This could be useful info for MD. Oximeter is very inexpensive.

Have you asked her why she's getting up from her bed to go sleep in her recliner, and if so what does she say?
As long as she's sleeping well, I'm not sure I would be concerned about where she's sleeping.
The night sweats sound like they may be heart related, and I hope you've talked to her doctor about those. Perhaps the night sweats are waking her up and she's having trouble breathing so she thinks if she's sitting up that she can breath better??? That's just a guess on my part.
So I would just ask her if you haven't already.