How Light Sensitivity Could be the Key to Understanding Sundowning in Alzheimer's

Families and friends of people with dementia know sundowning well. Sundowning is when symptoms of dementia get worse towards the end of the day. Researchers believe that disruptions in a person’s circadian rhythms might contribute to sleep disruptions and the progression of dementia and its most common form, Alzheimer’s. 

In other words, when we miss out on sleep, we also miss out on the brain’s ability to clear plaques in the brain that can eventually develop into Alzheimer’s. In someone with Alzheimer’s, continued sleep disruption can result in an increase in symptoms later in the day (although this doesn’t always occur right at sundown).

In a new study published in Frontiers in Aging Neuroscience, researchers have made significant strides in understanding the relationship between circadian rhythms and Alzheimer's disease. The research focused on two Alzheimer’s mouse models that simulate different aspects of the disease. The scientists aimed to compare the behavior of these models to normal mice concerning their circadian rhythms. 

Surprisingly, the Alzheimer’s mice displayed a substantially faster adjustment to changes in their circadian rhythms, similar to adapting to a simulated time zone change. 

One of the key discoveries of this study was that the faster adjustment observed in one of the types of Alzheimer’s mice was not affected by the activation or deactivation of microglia, which are immune cells that can protect or harm connections in the brain. This suggests that other factors, aside from microglia, contribute to the altered circadian behavior in AD model mice.

These same mice also showed increased sensitivity to low-intensity light cues. The researchers discovered more light-sensing neurons in the brains of these mice, suggesting that altered light sensing may contribute to their faster adjustments.

“These data suggest that controlling the kind of light and the timing of the light could be key to reducing circadian disruptions in Alzheimer’s disease,” said study author and University of Virginia School of Medicine’s Heather Ferris, MD, PhD in a news release. “We hope that this research will help us to develop light therapies that people can use to reduce the progression of Alzheimer’s disease.”