Study reveals impact of artificial light on sleep, metabolism

If you spend your workday under fluorescent lights, new research from Michigan State University suggests your body may notice. In a study of animals that, like people, are active during the day, findings revealed that living under fluorescent office lighting was linked to weaker daily rhythms, higher rates of diabetic-range blood glucose, and less active sperm than those living under daylight equivalent light. But a single hour of midday light helped make up the difference.

“As modern humans spend most of their time indoors, understanding how the quality of indoor light affects physiology may have important implications for the design of homes, schools, workplaces, and healthcare settings,” said Lily Yan, co-author of the study and professor in MSU’s Department of Psychology. “Light is not just something that helps us see. It is a biological signal that influences circadian organization and, potentially, a range of downstream health outcomes.”

The study, published in the Journal of Biological Rhythms, is the first to analyze how daylight impacts circadian rhythms and physiology by using programmable spectrum control to mimic real-world lighting environments and Nile grass rats, a species that is active during the day like humans. Nearly all lab rodents are nocturnal, which creates a mismatch when researchers want to understand how daylight shapes human biology.

In the first experiment, eight male grass rats cycled through four conditions for two weeks: steady daylight equivalent light, daylight that rose at dawn and fell at dusk, standard fluorescent office light, and office light plus a one-hour midday burst of daylight. In the second experiment, 20 males lived under either daylight equivalent or fluorescent lighting for six weeks. Body weight, blood glucose, sperm counts and sperm motility were tracked.

The study found that the grass rats living under daylight equivalent lighting showed the strongest daily rhythms, while the animals living in the office lighting showed the weakest daily rhythms. However, a single one-hour daylight exposure at midday partially restored the daily rhythms of the office lit grass rats.

In the second study, four out of 10 animals in the fluorescent group developed diabetic-range blood glucose, compared with one out of 10 in the daylight group. Sperm motility was less than half in the fluorescent group. Sperm counts were unchanged, but it did impair how well the sperm swam.

The team of interdisciplinary researchers, including architects, psychologists and physiologists, view these findings as promising early evidence, but they encourage other researchers to do additional research with grass rats or other diurnal animals to further determine whether similar effects occur in humans.

“Factors affecting health are not limited to drugs, diet and exercise. The light environments we create in our homes, schools and workplaces are also biological exposures,” said Antony Bum Kim, lead author of the study from U.C. Berkeley’s Department of Architecture. “Understanding their effect could help inform healthier indoor environments and identify practical, nonpharmacological approaches for supporting circadian and metabolic health.”