I never really had a decent explanation of what sundowning is, though the effects for Jan were clear.
One Harley Street consultant had recommended increasing the lighing levels at home and perhaps to put in one of those special fluorescent panels as she thought Jan might be suffering from SAD. This was way before she was formally diagnosed with dementia, and before the sundowning began [as far as I recall].
So often I had to experience and interpret what Jan was going through because no-one else seemed to have any idea, and quite inappropriate text book passages were quoted to me by medical people who seemingly had never experienced someone with dementia at first hand for any period.
My reading on the sundowning was that it was related to Jan sensing the time of day somehow. As this happened summer and winter, I don't think it can have been the light levels. She was as far as I know pretty oblivious to reading clocks by this time. It never happened in the mornings, only ever between 3pm and 6pm. Maybe I should have varied our routines more.
When she sensed the time was getting late, I think her body would be taken over by a natural homing instinct - she had to try and get wherever she thought home and safety was, before dark. Despite having lived in our house for 10-12 years, that was not home any longer, she was certain of that. Often she would say that her mother and father would be worried about her. They had died many years before.
For me, diversions worked only to a point. Only medication worked, and that had to be something that could be slipped into a drink. In the later stages even the subterfuges I used to encourage her to have the drink ["wow, I'm very thirsty! Do you want a drink as well?"] worked only sometimes.
I think the dread of knowing that sometime in the next hour the sundowning would probably begin was one of the worst things. It happened very quickly, and there were no easy early signs from Jan. I knew I had to get that drink into her as soon as possible, and that it would take about half an hour to work. Until then I would have to batten down my hatches and accept whatever happened.
It is worth trying anything that is safe for the patient to alleviate the worst parts of sundowning, because it is sheer torture for them as well.