Why does my mom's demeanour change at bedtime?

My 99-yr old aunt with mod/adv dementia does the same thing. Mostly awful all day (especially during sundowning) but as soon as I was putting her in bed at night she'd go on and on thanking me (and the other family caregivers) and acknowledging how hard it must be. It's our glimpse of her prior self every single night. And we're so grateful for that mercy.

You don't mention in your profile that Mom has Dementia but you do say she is confused so I will assume she does. I think Grandma's explanation is a good one. Those suffering from dementia can get over stimulated. My Mom, every so often would close her eyes. Sit and talk to you, all the time keeping her eyes shut. And when she did this, she made more sense than she did when her eyes were open.

There could be so many reasons and without your mom actually telling you...and you know that won't happen...and on a conscious level she may not even know she is doing this. But here are some maybes...
At bed time she is tired and is allowing he brain to just wander and "be itself" rather than trying to "hold it together" or process all that is going on around her.

Sundowning usually causes negative mood swings. It might be possible that your moms brain shifts the other way and she becomes m ore complacent.

At bedtime things are winding down, she is less stimulated by activity around her less noise, maybe the light is a bit dimmer so she can focus just on you. And likewise you are focused just on her. A little bit of "girl time".