I agree with everything everyone has said.
Check meds, (and not just meds, but certain meds interact with certain foods causing all sorts of weird things).
All that said, after several years of doing the nursing home thing, and 7 years of hospice - Sundowner, aside from any interaction of meds, or foods, or even Alzheimers - is, indeed a real phenomenon. And, I call it a phenomenon, because, caretakers have seen it for years, and NOT just in Alzheimers cases. (of course, if you are a nurse, doctor's and scientists say whatever they will say, and figure that you are not educated enough to see what you see).
Some patients - aside from the Alzheimers diagnosis, especially older folks who have had several other medical problems, can become irrational,
hallucinate, and can even become dangerous (violent) by the end of the day - (why its called Sundowner's). Nurses have long realized that it helps a lot to get patients to sleep earlier, or to get them to nap during the day - and that with those person's who do not have Alzheimers (or other dementia), can quite often relieve some of the symptoms. A common thread is getting patients up so darned early. Even though it is well documented that a "regiment" can help considerably, sometimes the structure is way TOO structured to a population rather than an individual. Some people's biological clocks are different than others, and when you take some people who have always been "night owls", put them on a strict regiment that gets them out of bed at 6-7 am, makes them eat by 8 am, makes them eat again at 12:30 pm, dinner at 5 pm, and bed by 9, does NOT reset their biological clock, but rather makes the situation worse. Some of the newer (and better) nursing homes are now putting tv's and radios in individual rooms (more like a bedroom or a hospital room), where a patient can continue what feels natural and familiar, helps soften symptoms of Sundowners.
As does natural light. More places are going with the lighting that has the components of sunlight, and, amazingly, in those instances, depression and Sundowners have improved dramatically. You can buy one yourself. It is the nursing home's version of "cabin fever" (the real reason for cabin fever is lack of sunlight-its a real condition).
My father-in-law's father was having such an issue, and he did not have any disease in the Alzheimers or dementia areas, but, by evening, you could not even talk to him. The nursing home had no problem with us changing his light bulbs, to see if it would help. It did. Now, things didn't become "rosy" of course, he had other issues, but, it sure made everyone a little better - within a year, the nursing home started using them in several places at the facility.
Also, it is never, never, never, easy to take care of someone else. It just isn't. And, it is not easy for a person who has taken care of themselves for years, to be at the mercy of a bunch of other people - even if its their own children. So, after you take care of all the obvious (meds, food, combinations of the meds or meds and food) - you should start looking at the other issues. Oh, and if there is a respiratory disease faction in a person, you might also want them to make sure that their O2 levels are up there, and their CO2 levels are low - that also creates what LOOKS like Alzheimers or Sundowners.
Beth