Is something going on with my 91-year-old mother?

In ALZ hallucinations tend to come later in the disease progression. In Lewy Body Dementia hallucinations may be the first sign that something is wrong. Hallucinations have other causes besides dementia, but they are not a normal part of aging.

Delusions and hallucinations are common with Alzheimer's and other forms on demensia. In stages 5-6 my mom always saw a little boy who was lost. She would tell us she was worried about him and where did he go. We told her we called his mom and dad and they came and picked him up. That brought comfort. If your mom is wanting the cat to sit with her, get a stuffed cat. We had a stuffed dog for my mom. She could hold it on her lap. It brought a lot of comfort for months.

You may want to have your mom was evaluated by a neurologist or geriatric psychiatrist.

Does she have a cat? Either way, obsessing on a particular topic can be a sign of mental health issues. By itself it is fairly harmless, except that it drives loved ones up a wall!

Having delusions and/or hallucinations of visitors is not typical age-related decline. Something else is going on. Talk to her primary care provider.

If having a light on at night provides her comfort, why not?