Researchers already know that people who stay up late watching TV, playing video games or surfing the Web have an increased risk of becoming obese. The thought was that all that sedentary awake time amounted to more opportunities to snack, and thus to put on pounds.

But a study published online Monday says another mechanism may be at work too – the additional exposure to light at night messes with one’s circadian rhythm and throws the body’s metabolism out of whack.

To prove that light at night actually causes obesity, researcehrs from Ohio State University and University of Haifa in Israel kept three groups of mice. One group followed the normal pattern of 16 hours of full light and eight hours of dark; another group had 16 hours of full light and eight hours of dim light; the third group was exposed to full light for 24 hours in a row.

All three groups of mice ate the same amount of chow, and they all got the same amount of exercise. However, after just one week, the mice in the two groups that never experienced total darkness weighed significantly more than the mice who had a normal light-dark schedule. The gap persisted over the eight weeks of the study, and by the end they weighed about 10% more.


Extra grams weren’t the only issue. Halfway through the experiment, the dim-light and full-light mice also experienced “impaired glucose tolerance,” according to the study, which was published in Proceedings of the National Academy of Sciences.

One other interersting finding – the mice who had a normal light-dark schedule ate only 37% of their food during the light phase, but the mice who only got the lights dimmed at “night” ate 56% of their chow during the light phase. Since mice are nocturnal, this was the equivalent of nighttime snacking.

-- Karen Kaplan/Los Angeles Times