Light at night, what a fright!

When I picked my paper for this week’s blog, a very recent article published in PNAS, I didn’t factor in that I would write it on Halloween. Now I realize it’s going to seem like some cruel joke: on the one night where people stay up late walking the streets with flashlights and eat candies, I’m writing about the link between light at night and obesity. Wow. If I had tried to pick something more fitting, I couldn’t have done a better job.

We all know obesity is on the rise and there are several reasons to explain the epidemic: increased intake of calories (Double Down sandwich, anyone?), dietary choices (cheezy poofs instead of apples, anyone?) and lack of exercise (reading blogs, anyone?). However, the rise of obesity rates also coincides with an increase in light at night – the artificial lighting that allows us to write blogs late at night and catch up on all other activities we didn’t have time to do during the daytime. The problem is that light is closely tied to our circadian rhythm (the built-in clock that controls our biological processes and our behaviour). When our circadian rhythm is disrupted (an extreme example of this is shift workers), so is our metabolism. Based on this logic, an international team of researchers set out to test whether light at night plays a role in the weight of mice.

The researchers divided their mice into three groups. The control group was housed in the standard light/dark cycle. Another group of mice was housed in a light/dim light cycle (let’s call them the “dim” group). Finally, a third group of mice was housed in a continuously lit room (let’s call them the “bright” group). The mice were housed in these conditions for eight weeks, and during this time, the researchers monitored a number of parameters including body mass, food intake, activity levels, and glucose tolerance (how quickly sugar is cleared from the blood).

The researchers found that all the mice that experienced light at night (both the dim group and the bright group) got significantly fatter than the control mice. What’s more, the dim group and the bright group also exhibited impaired glucose tolerance (this can mean the mice are in a prediabetic-like state). Did the light at night groups of mice eat more (who doesn’t get the munchies when watching a late-night movie)? No. Did they exercise less (who goes for a run at midnight)? Also no. So what happened?

As it turns out, while the two light at night groups of mice ate just as much as the control mice, they ate at different times. Mice are nocturnal animals, and so normally they do most of their eating at night. The mice in the dim group, however, ended up eating over half of their food during the “light” phase. When the mice in the dim group were forced to eat their normal food intake only during the normal (dark) time, they didn’t gain weight. How crazy is that? These results suggest that light at night disrupts the timing of food intake, and this throws the metabolism out of whack.

Anyone who has looked up weight loss tips knows that it’s a good habit to forgo eating past a certain time of night (usually 7 or 8pm) if you want to lose weight. The reason usually given to explain this is that night time food is most often unhealthy and calorie-laden snacks: munchies during a movie, or ice cream after a distressing phone call from the ex-boyfriend. This study suggests there might be something more to this weight loss strategy: it may be all about listening to our biological clock.

Reference: Light at night increases body mass by shifting the time of food intake. (2010) Fonken LK et al. PNAS 107(43):18664-9.