Study Sheds Light on How Darkness Produces Dimwits

The structure of the brain, and even the ability to learn and remember knowledge, may be hampered by spending too much time in dim light, researchers at Michigan State University have concluded, in a just-published, first-of-its-kind study likely to reinforce why bright is a synonym for smart.

The researchers exposed Nile grass rats to dim and bright light for four weeks, studying the effect on the rodents’ brains. The rats – which like humans are diurnal and sleep at night – lost about 30% of capacity in their hippocampus and performed poorly on a spatial task on which they had previously trained following exposure to dim light.

Rats exposed to bright light, however, showed significant improvement on the spatial task. And when the rodents that were 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 both recovered fully.

Researchers detailed these and other results in “Light Modulates Hippocampal Function and Spatial Learning in a Diurnal Rodent Species: A Study Using Male Nile Grass Rat (Arvicanthis niloticus),” which they said was the first study to show that changes in environmental light, in a range normally experienced by humans, leads to structural changes in the brain.

The study was published in the journal Hippocampus.

Potential Site in Brain under Study

After passing through the eyes, light acts first on other sites within the brain and does not directly affect the hippocampus. Lily Yan, M.D., Ph.D., associate professor of psychology at Michigan State and the study’s principal investigator, said the research team is studying one potential site in the rodents’ brains—a group of neurons in the hypothalamus that produce orexin, a peptide known to influence a variety of brain functions.

Among researchers’ major research questions: If orexin is given to the rats that are exposed to dim light, will their brains recover without being re-exposed to bright light?

“For people with eye disease who don’t receive much light, can we directly manipulate this group of neurons in the brain, bypassing the eye, and provide them with the same benefits of bright light exposure?” Dr. Yan asked. “Another possibility is improving the cognitive function in the aging population and those with neurological disorders. Can we help them recover from the impairment or prevent further decline?”

Researchers reported that sustained exposure to dim light led to significant reductions in brain-derived neurotrophic factor. This peptide helps maintain healthy connections and neurons in the hippocampus, as well as in dendritic spines, or the connections allowing for communication between neurons.

“Since there are fewer connections being made, this results in diminished learning and memory performance that is dependent upon the hippocampus,” added Joel Soler, a Ph.D. graduate student in psychology and the study’s lead author. “In other words, dim lights are producing dimwits.”