Scientists have new evidence that “sundowning” — late-day anxiety and agitation behaviors exhibited by older institutionalized adults — has a neurological basis, according to a new study.

Sundowning is usually seen in dementia patients and is characterized by symptoms of agitation, anxiety, general activity and delirium prior to going to bed. It can be challenging for caregivers to deal with since patients can become aggressive.

“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,” said Tracy Bedrosian, author of the study and doctoral student at The Ohio State University.

Bedrosian and her colleagues say they have observed sundowning behaviors in aged mice in the hours before they would typically sleep. Like humans, they demonstrate more anxiety-related activities and are much more agitated. Mice engineered to have Alzheimer’s-like conditions displayed more anxiety before sleep, too.

“Some people have argued that sundowning could be explained just by a buildup of frustration in older people who couldn’t communicate their needs over the course of the day, or by other factors,” said the study’s co-author, Randy Nelson, Ph.D. “But our findings suggest there is a real phenomenon going on here that has a biological basis.”