My dad has sundowners syndrome and some days it's 12 hours. What should I do?

Have you talked to his doctor about this? If his doctor doesn't know that there are medications that can help with agitation, delusions and hallucinations, perhaps you can find a geriatric psychiatrist who has experience using this medications to help with the sundowning.

Have you had him checked for a UTI? Definitely talk to his doc. There may be something else causing this. Is his doc a geriatrician or neurologist specializing in dementia? A family care doc will not have the background to be able to help dad.

....... After a month of dealing with sundowning from 11am on post 4th stroke we had a break though. My MIL/GMIL takes 25 mg of sereqaul in the morning and 100mgs at night and we are doing GREAT! She now is her self roughly until around 4-5pm (then gets into fights with the ceiling fan or the TV) but we give her night time meds at roughly the same time so have about 30-45 min of confusion now instead of hours and hours. It is a fine balance to see what works for each person.