A report published online this week in the Proceedings of the National Academy of Sciences suggests that some of the increase in obesity observed over the past several decades could be due to increased exposure to light at night and shift work, which disrupts the release of melatonin, a hormone produced by the pineal gland in the brain in response to darkness. "Light at night is an environmental factor that may be contributing to the obesity epidemic in ways that people don't expect," observed study coauthor and Ohio State University professor of neuroscience and psychology Randy Nelson. "Societal obesity is correlated with a number of factors including the extent of light exposure at night."
Dr Nelson and his colleagues evaluated the effects of nighttime light exposure in 8 week old male mice. The animals were exposed to 24 hours of constant light, to a standard light-dark cycle consisting of 16 hours of light and 8 hours of darkness, or to 16 hours of daylight and 8 hours of dim light which models environmental light pollution experienced by those living in industrialized nations.
As early as one week after the onset of treatment, a greater increase in body mass was observed among mice exposed to constant light and those exposed to a cycle of daylight and dim light compared to animals that received standard light-dark exposure. At 4 weeks these groups showed impaired glucose tolerance. Mice exposed to the standard light-dark cycle gained 8 grams of weight by 8 weeks and had less epididymal fat pad mass gain compared to the other mice, whose weight gain averaged 12 grams. "Although there were no differences in activity levels or daily consumption of food, the mice that lived with light at night were getting fatter than the others," noted lead author and OSU doctoral student Laura Fonken.
In another experiment, mice exposed to a standard light/dark cycle or a cycle of daylight and dim light were provided with continuous access to food or access during the light or dark phase. Feeding during darkness prevented excess weight and fat gain in animals exposed to the daylight/dim light cycle. "Something about light at night was making the mice in our study want to eat at the wrong times to properly metabolize their food," Dr Nelson stated. "When we restricted their food intake to times when they would normally eat, we didn't see the weight gain. This further adds to the evidence that the timing of eating is critical to weight gain."
Due to their nocturnal nature, mice normally consume most of their food at night when they are active, in contrast with humans, who normally eat during the day. Dr Nelson suggests that in humans, late-night eating could be a risk factor for obesity. Exposure to light at night disrupts melatonin signaling, which could lead to changes in food intake and activity, and altered metabolism.
"It may be that people who use the computer and watch the TV a lot at night may be eating at the wrong times, disrupting their metabolism," Dr Nelson said. "Clearly, maintaining body weight requires keeping caloric intake low and physical activity high, but this environmental factor may explain why some people who maintain good energy balance still gain weight."