To avoid gaining weight, suggests a new study, we may need some darkness.
Mice exposed to light at night gained 50 percent more weight than mice whose nights were truly dark, the study found, even though the two groups ate equal amounts of food and got the same amount of exercise.
What seemed to matter most was the timing of their meals. With exposure to nighttime light, the study found, mice ate at times when they normally would be sleeping. That alone could have altered their circadian rhythms and led to weight gain. Dim lights had the same effect as bright ones.
While the mice in this study were nocturnal, the same could prove true in people. That would emphasize the need to shut the shades and turn off lights at night, including computers, TVs and smart phones -- not just to help with sleep, but to help us keep our figures.
"With the advent of electrical lighting at the turn of the 20th century, individuals of many species, including humans, became exposed to bright and unnatural light at night," said lead researcher Laura Fonken, a behavioral neuroscientist at The Ohio State University in Columbus. "Our findings are important because they demonstrate how modern societal developments can impact health."
Fonken and colleagues noticed that rates of obesity have risen alongside a rise in light levels at night. They wondered if the parallel trends might be more than a coincidence.
The researchers conducted two experiments. In the first, they exposed one group of mice to 16 hours of light during the day and eight hours of complete darkness at night. A second group of mice experienced the same daytime light exposure with eight hours of dim light -- at a level that was like having a computer on in the room. A third group got 24 hours of bright light.
After eight weeks of being able to eat as much as they wanted, all three groups gained weight. But mice that had been exposed to light at night gained an average of 12 grams, compared to a gain of eight grams in the mice whose nights were dark. The light-exposed mice also had greater trouble regulating blood sugar levels.
All of the mice, which were nocturnal, consumed about the same number of calories and got the same amount of physical activity. But those exposed to light at night ended up eating 56 percent of their calories during daytime hours, the researchers report today in the Proceedings of the National Academy of Sciences. Those that experienced true darkness ate just 37 percent of their calories during the day.
To test the idea that timing of meals might make a difference in weight gain, the researchers again exposed groups of mice to the three light scenarios. This time, meals were restricted to times of day when the animals normally eat. In that case, mice didn't gain weight, even if they experienced late-night lights.
"I think this has really important implications for weight control and how we approach weight and diet," said neurologist Phyllis Zee, director of the Sleep Disorders Program at Northwestern University in Chicago. "Maybe this should be a bit of a wake-up call when we're looking at weight-management strategies -- to think about timing."
The study could be an important insight in the fight against obesity in young people, Zee said, because teenagers naturally tend to fall asleep later and are fiends for their electronic devices. The findings could also help explain why shift-workers suffer such high rates of obesity, diabetes and other metabolic disorders.
For everyone, Zee said, the new study underlines the need for good sleep habits. Around 9:30 p.m., she said, it's a good idea to shut off electronics, dim overhead lights and start winding down.
"Now we have another reason to pay attention (to good sleep hygiene)," she said. "Not only because it can help you sleep better, but because it can also help us maintain our weight."