Too much light may be making you sick, say researchers at Leiden University Medical Center in the Netherlands. An excess of light throws your body clock out of whack and increases biomarkers for inflammation and other conditions associated with aging.
Mice kept under conditions of constant light for a period of months showed many negative impacts on their cells' internal clocks — the sleeping and waking cycles called circadian rhythm, which influences the release of hormones, body temperature, and other bodily functions.
"Our study shows that the environmental light-dark cycle is important for health," says researcher Johanna Meijer. "We showed that the absence of environmental rhythms leads to severe disruption of a wide variety of health parameters' pro-inflammatory activation."
Those parameters cause an increase in inflammation in the body, muscle loss, and early signs of osteoporosis. The physiological changes were all indicative of "frailty" that is typically seen in people or animals as they age.
But there was also good news. "The good news is that we subsequently showed that these negative effects on health are reversible when the environmental light-dark cycle is restored," Meijer says.
For the study, mice were exposed to light around the clock for 24 weeks. Researchers found that  the constant light exposure reduced the normal rhythmic patterns in the brain's circadian rhythm by 70 percent.
The disruption in light and dark cycles led to a reduction in the animals' skeletal muscle function as measured in standard tests of strength. Their bones showed signs of deterioration, and the animals entered a pro-inflammatory state normally observed only in the presence of pathogens or other harmful stimuli.
When researchers returned the mice to a standard light-dark cycle for two weeks, their circadian rhythm normalized and their health problems were reversed.
The findings suggest that people, especially those who are aging, should examine the amount of light exposure they are getting, and make sure they get sufficient "dark" time by turning out the lights. 
"We used to think of light and darkness as harmless or neutral stimuli with respect to health," Meijer says. "We now realize this is not the case."
The bottom line, according to the researchers is "light exposure matters."