Older adults with dementia experience sundowning syndrome - characterized by high levels of anxiety, agitation.
Some people have argued that the syndrome could be explained just by a buildup of frustration of older people who couldn't communicate their needs over the course of the day, or by other factors.
But new findings suggest there is a real phenomenon going on here that has a biological basis.
"It's a big problem for caregivers. Patients can get aggressive and very disruptive," said Tracy Bedrosian, lead author of the study and a doctoral student in neuroscience at Ohio State University.
"There have been a few clinical studies documenting sundowning, but until now there hasn't been research in animals to see what's going on in the brain to explain this," she added.
The new study found that aged mice showed significantly more activity and more anxiety-like behaviors in the hours before they would normally sleep when compared to middle-aged mice - just like sundowning in humans.
In these aged mice, the researchers found changes in parts of their brain associated with attention, emotions, and arousal, all of which could be associated with the behavior seen in sundowning.
In addition, mice that were genetically engineered to have an Alzheimer's-like disease also showed more anxiety before sleep than did other mice.
The study will appear in the online Early Edition of the Proceedings of the National Academy of Sciences.