Quick summary
Sundown syndrome is a term that describes the onset of confusion and agitation that generally affects people with dementia or cognitive impairment and ususally strikes around sunset. Many people, though, use the term to loosely describe increased agitation and confusion that can occur anytime but may be more noticeable in the late afternoon or early evening.
Although researchers equate sundown syndrome with dementia, people without dementia sometimes develop delirious and agitated behavior in the hospital as a reaction to pain, medical procedures, or infection.

While I was changing planes in Atlanta, I was asked by an elderly gentleman to please talk with his wife, who had Alzheimers and was having sundown. The husband and wife were supposed to catch a plane to Chicago, to meet their daughter. The wife had insisted on getting out of her wheelchair, to sit in an airport chair, that was halfway to their gate. The wife refused to go to the gate to get onto their plane. She said that their daughter would come to Atlanta to meet her. The airline employees said that they would never force a person to board a plane against their will. (Which I can understand.) The husband tried to reason with his wife, which of course did not work. I had to catch my plane, so I did not see the end. My mother now has Alzheimers and my father has Parkinson's. What can a care giver do in a simular situation, when the person with sundown will not move, when it is necessry?
My mother-in-law has had 3 bad spells of hallucinations, after those 3 days she calmed back down and hasn't had anymore since. They were always after sundown. Is this common for them to come and go like that?