My grandma (95) is convinced mice are in her bed. She's also convinced she has fleas. How do I address her fears?

You cannot convince a person with dementia who's sundowning of anything, frankly, so all you can really do is divert her attention elsewhere. Until she goes back to the fleas and the mice, and then you'll distract her again. And on and on the story goes, ad infinitum, unless her doctor prescribes an anti-anxiety medication for her, and even then, the delusions can still persist. You can read the following link for suggestions about how to deal with sundowning, but again, at 95, there isn't much aside from calming meds that are likely to help your grandma at this stage of the game:

https://dailycaring.com/7-ways-to-reduce-dementia-sundowning-symptoms/

It's unfortunate that your gma's doctor can tell you she's sundowning, but not what to DO about it, huh? Sad too that most doctors don't know a darn thing about the brain or how it functions or stops functioning when certain dementias strike. My mother is 94 and lives in Memory Care. I'll be darned if I can talk her off the various ledges she's on every night, carrying on about the same things, too. No matter what I tell her, she has certain ideas in her head that won't be chopped out of there with a chisel! Sad but true. I distract her to the best of my ability, and then suggest she put on the tv and watch something silly to make her laugh. Maybe you can put on a favorite comedy for your grandma to watch when she gets talking about bugs? Nothing like a bit of laughter to diffuse an otherwise stressful situation.

Best of luck

Dont't know that this will help but you might try it. Mice always leave behind evidence with their droppings. One would have to have the speediest single mouse in order to have "A" mouse issue. That is not likely. Ask her to show you droppings. Again not likely but maybe give it a try. I have not had fleas but I would be honest here if I did. From what I know fleas bite thus leaving bite signs which itch and likely bleed. I know it is very hard to reason with a compromised elderly person but since you are being deluged with these specific issues I think you could at least give this a try.

Seeing things is a symptom of dementia. Mom was convinced lizards were lurking in the corners. How about a fairy mister? Fill a clean spray bottle with water, add scent if you like, and use it to "kill" the fleas and mice.