“Light is not just something that helps us see,” said Lily Yan, co-author of the study and a professor in MSU’s Department of Psychology.
Working under fluorescent office lights may weaken daily rhythms, raise blood glucose into diabetic ranges, and slow down sperm movement, according to new research from Michigan State University reported on November 11, 2026.
Published in the Journal of Biological Rhythms, the research evaluated Nile grass rats, an animal species active during the day like humans. The study, titled Daylight, Daily Rhythms, and Downstream Physiology: A Translational Study in Diurnal Nile Grass Rats ( Arvicanthis niloticus ), was authored by Kim et al. in 2026.
Scientists tracked body weight, blood glucose, sperm counts, and sperm motility across different lighting conditions over periods of two to six weeks, as described by Medical Xpress.
Nearly all lab rodents are nocturnal, creating a mismatch when researchers study how daylight shapes human biology, which led this interdisciplinary team of architects, psychologists, and physiologists to use Nile grass rats. By using programmable spectrum control to mimic real-world lighting environments, the project first analyzed how daylight affects circadian rhythms and physiology.
In the initial experiment, eight male grass rats cycled through four distinct 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.
The study found that grass rats kept under steady daylight-equivalent lighting maintained the strongest daily rhythms. Animals housed under standard fluorescent office lighting exhibited the weakest daily rhythms.
In a second experiment tracking 20 male rats over six weeks, four out of ten animals in the fluorescent lighting group developed diabetic-range blood glucose. In contrast, only one out of ten animals developed it in the daylight-equivalent group.
Sperm counts remained unchanged between the two groups. However, sperm motility in the fluorescent group dropped to less than half of the daylight group’s level, impairing how well the sperm swam.
“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,” Yan told Michigan State University. She added that light serves as a biological signal influencing circadian organization and potential downstream health outcomes.
Exposing office-lit grass rats to a single one-hour burst of daylight at midday partially restored their daily rhythms during the first experiment.
While viewing their findings as promising early evidence, the researchers encourage additional studies with grass rats or other diurnal animals to determine if similar effects occur in humans.
“Factors affecting health are not limited to drugs, diet and exercise,” said Antony Bum Kim, lead author of the study from U.C. Berkeley’s Department of Architecture.
“The light environments we create in our homes, schools and workplaces are also biological exposures,” Kim told Medical Xpress, noting that understanding these effects could help inform healthier indoor environments and identify practical, nonpharmacological approaches for supporting circadian and metabolic health.
