My mom with dementia has been sleeping more and more. Last night she slept from 6PM to 6AM this morning not even to go to the bathroom.

Assuming her BP checks out okay, it could be just the stress and exhaustion of the ER visit which likely took all day............that takes a lot out of an elder! Sleeping 12 hrs isn't really overly excessive anyway, so try not to worry too much.

Why did you take your mom to the ER?

Since being on the new med have you taken her BP? New BP medication isn’t always the correct dosage. Sometimes (often) it has to be adjusted. Keep a log of her BP and O2 and Pulse. It will give you enough information to be helpful with how her body is responding to the new meds.

What is her normal sleep pattern? I assume it’s much less than 12 hours based on your reaction.

Usually the ER doctor expects a patient to follow up with their GP after an ER visit. Is your mom scheduled to see her GP?

I would get her BP checked. Maybe it's too low. Or it could certainly be related to her dementia. Sleeping 12 hours isn't a totally crazy amount of sleep.

Any change in mental status is supposed to be reported to the doctor. Does she have other medical conditions? Did the ED check her for things like UTI, flu, or some other illness? How old is she? Is she normally alert?