Spending too much time in dimly lit rooms and offices may actually change the brain's structure and hurt one's ability to remember and learn, indicates groundbreaking research by Michigan State University neuroscientists.

The researchers studied the brains of Nile grass rats (which, like humans, are diurnal and sleep at night) after exposing them to dim and bright light for four weeks. The rodents exposed to dim light lost about 30 percent of capacity in the hippocampus, a critical brain region for learning and memory, and performed poorly on a spatial task they had trained on previously.

The rats exposed to bright light, on the other hand, showed significant improvement on the spatial task. Further, when the rodents that had been exposed to dim light were then exposed to bright light for four weeks (after a month-long break), their brain capacity  and performance on the task recovered fully.

The study, funded by the National Institutes of Health, is the first to show that changes in environmental light, in a range normally experienced by humans, leads to structural changes in the brain. Americans, on average, spend about 90 percent of their time indoors, according to the Environmental Protection Agency.

When we exposed the rats to dim light, mimicking the cloudy days of Midwestern winters or typical indoor lighting, the animals showed impairments in spatial learning.

Interestingly, light does not directly affect the hippocampus, meaning it acts first other sites within the brain after passing through the eyes.