Leaving Night Light On Could Cause Weight Gain

Keeping a light on at night could affect your eating schedule and add a few inches to your waistline, a new study in mice suggests.

Mice exposed to a dim light at night gained 50 percent more weight over an eight-week period than mice that spent their nights in total darkness, said study researcher Laura Fonken of Ohio State University.

"In many ways, our society now functions on a 24-hour-a-day schedule," Fonken told MyHealthNewsDaily. "These results suggest that such a schedule may impact metabolic function," leading to weight gain.

The findings held up even when the amount of food and the physical activity of the mice were held constant, researchers said, and the results could apply to people who eat meals late at night.

Other studies have found that working the night shift can cause poor health.

And previous work has shown hormones that aid in metabolism are affected in humans exposed to light at night, Fonken said. Night light could have reduced those hormones in mice, and coupled with a disruption in the mice's internal clocks, could have been responsible for their weight gain.

Can light make you heavy?

In the study, mice subjected to 16 hours of daylight and eight hours of dimmed light gained 0.4 ounces (12 grams) of body mass, whereas mice subjected to 16 hours of daylight and eight hours of darkness gained 0.3 ounces (8 grams) of body mass. The researchers fed all mice the same amount of food, and observed no difference in how much they moved around their cages.

The mice exposed to the dim light at night also had higher levels of glucose intolerance, which is a marker for pre-diabetes, than the mice that had complete darkness.

Mice are nocturnal creatures so they would normally eat at night, researchers said. But mice that lived in the dim-light conditions ate 55 percent of their food during daylight, compared with the mice in the standard light-dark conditions that ate 36 percent of their food in daylight, the study said. That shows their internal eating schedules were set off-kilter by the light changes, Fonken said.

"Mice exposed to light at night showed altered feeding behavior, eating more during the rest phase, which may be responsible for the increased weight gain," she said.

Timing is key

The findings support a number of other recently published studies about the link between weight gain and internal clocks.

One study published last year in the journal Obesity looked at how the off-setting of the internal clock can affect weight. Mice that were fed meals during their normal sleeping hours added 48 percent to their body weight over six weeks, whereas mice fed meals during normal times gained 20 percent of their weight.

A 2009 study in the journal Proceedings of the National Academy of Sciences showed that working the night shift increased the risk of obesity, in addition to heart disease and diabetes.

However, a small 2006 study in the journal Obesity Research showed that late-night snacking did not lead to weight gain in monkeys. But this study looked at snacking only, so its implications may not apply to consumption of whole meals during late-night hours, the researchers said.

Next, researchers want to replicate the new study in humans to see if the phenomenon holds true, Fonken said.

The study was published online today (Oct. 11) in the journal Proceedings of the National Academy of Sciences.

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