It’s because people with Alzheimer’s become disoriented very easily as it progresses. And there are way more visual cues for time of day (sunrise vs mid day vs sunset) and season of the year (snow, bare trees, green leaves vs brown/red leaves, etc.) when it’s daytime. At night, they don’t know where they are, what time it is, or what day/time of year it is because they don’t have as many clues to help them.