Dim light can make you dumber. Being in low light during the day produces changes in the brain that can interfere with learning and memory. People who spend a lot of time in dimly-lit rooms might take some comfort from the fact that the study that found this connection was done with Nile grass rats, not people. But it still suggests that you may do your best work — and learning — in well-lit rooms.

These particular rodents are closer to humans than most because they are diurnal — they are more active during the day than at night. It only took four weeks of dim lighting for the physical and chemical changes in the brain to show up. Perhaps more important, however, was the evidence the researchers uncovered that showed these rats had lost some of their ability to remember and learn.

In one area of the hippocampus, the number of dendritic spines, fine hair-like projections that connect one brain cell to another, decreased by 30 percent.

The study looked at two groups of Nile grass rats housed in a laboratory. During the day, one group lived in dim light — 50 lux, a light level a bit lower than that found outdoors on a cloudy day. The other group received much brighter light during the day, 1,000 lux.

After four weeks, rats exposed to dim light performed poorer in the Morris Water Maze Task, a test where the rats are placed in a circular pool and need to find their way to a platform that lets them escape. The test is widely used by neuroscientists to study spatial learning and memory.

These rats also showed physical changes in their brain's hippocampus, a region intimately involved in long-term memory storage and spatial navigation. In one area of the hippocampus, the number of dendritic spines, fine hair-like projections that connect one brain cell to another, decreased by 30 percent. Fewer connections in an area of the brain indicate a lack of complexity and plasticity, which in turn suggests a decreased capacity for learning and memory storage.

Rats living in dimly-lit conditions also showed a decrease in a substance called brain derived neurotrophic factor (BDNF), a protein that helps keep nerve cells growing and functioning properly and is important to learning and memory.

On the positive side, the rats seemed to recover after four weeks of exposure to bright light. Both dendritic spine number and BDNF level increased, suggesting that the negative effects of dim light are only temporary and can be reversed.

Nile grass rats are not people. They live in underground burrows, and they have faced different evolutionary pressures. When they come up to the surface, they have to pay a great deal of attention to their surroundings to make sure that they don't get eaten by predators. Humans, on the other hand, mainly need to avoid salespeople, bill collectors and traffic.

Future research will hopefully determine whether these changes also happen in the human brain when a person spends a lot of time exposed to dim light. In the meantime, people have another reason to get out and spend a little time in the sun.

The study appears in Hippocampus.