Spending too much time in dimly lit rooms and offices may change the brain's structure and reduce people's ability to remember and learn, a study has found.
Researchers from Michigan State University (MSU) in the US studied the brains of Nile grass rats after exposing them to dim and bright light for four weeks.
The rodents exposed to dim light lost about 30 per cent 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, published in the journal Hippocampus, is the first to show that changes in environmental light, in a range normally experienced by humans, leads to structural changes in the brain.
"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," said Antonio Nunez, psychology professor at MSU.
"This is similar to when people can't find their way back to their cars in a busy parking lot after spending a few hours in a shopping mall or movie theater," said Nunez.
Sustained exposure to dim light led to significant reductions in a substance called brain derived neurotrophic factor - a compound that helps maintain healthy connections and neurons in the hippocampus - and in dendritic spines, or the connections that allow neurons to "talk" to one another.
"Since there are fewer connections being made, this results in diminished learning and memory performance that is dependent upon the hippocampus," said Joel Soler, a doctoral graduate student at MSU.
"In other words, dim lights are producing dimwits," he said.
Light does not directly affect the hippocampus, meaning it acts first other sites within the brain after passing through the eyes.
The research team is investigating one potential site in the rodents' brains - a group of neurons in the hypothalamus that produce a peptide called orexin that's known to influence a variety of brain functions.
The project could have implications for the elderly and people with glaucoma, retinal degeneration or cognitive impairments.
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