THE blame for rising obesity rates has been pinned on many things, including a more calorific diet, the rise of processed food, a lack of exercise and modern man's generally more stressful lot. Something else may soon be included in the list: brighter nights.

Light regulates the body's biological clock—priming an individual's metabolism for predictable events such as meals and slumber. Previous research has shown that, in mice at least, the genes responsible for this can be manipulated so as to make the animals plumper, and more susceptible to the associated problems of diabetes and heart disease. It was not known, though, whether simply altering ambient light intensity might have similar effects.

A team of researchers led by Laura Fonken of Ohio State University in America has cleared the matter up. As they report in Proceedings of the National Academy of Sciences, they examined how nocturnal light affects the weight, body fat and glucose intolerance (the underlying cause of late-onset diabetes) of male mice. They found that persistent exposure to even a little night-time light leads to increases in all three.

To reach this conclusion Dr Fonken split her murine subjects into three groups. Some were kept in cages lit constantly, so as to resemble a never-ending overcast day. A second group lived in conditions akin to their natural habitat, with 16 hours of overcast day-like light, followed by eight hours of darkness. The remaining rodents were also exposed to a cycle, but the dark was replaced with a dim glow equivalent to the twilight at the first flickers of dawn.

Over the eight-week period of the experiment the mice in the first and third groups gained almost 50% more weight than those exposed to the natural light-dark cycle. They also put on more fat and exhibited reduced tolerance of glucose, despite eating comparable amounts of food and moving around just as much.

The only thing that seemed to differ was when the mice ate. In the wild, mice are nocturnal critters. Unsurprisingly, then, those in the quasi-natural conditions consumed only about a third of their food in the “day” phase. For a mouse exposed to the twilit cycle, however, the figure was over 55%.

In a follow-up experiment, Dr Fonken looked at whether the timing of food consumption alone could explain the observed differences. It turned out that those forced to eat during the “day”, ie, out of whack with their biological clock, did indeed gain about 10% more weight than those fed at “night” (be it dark or just dim) or those with uninterrupted access to grub.

How this might relate to people will require further investigation. Mice and humans are physiologically alike, so a similar effect might be expected for people, but the fact that mice are nocturnal and humans diurnal is a serious complicating factor. It is true, though, that the spread of electric lighting means many people eat their main meal of the day in the evening, when natural daylight is long gone—the obverse of a mouse eating during daylight hours. And that tendency to eat late, though it has never been tested properly, is believed by many nutritionists to be a factor in putting on weight.

When the full explanation for the modern epidemic of obesity has emerged, it is unlikely that the spread of artificial lighting will be the whole of it. But this work suggests it might be a part. When you eat could be as important as what you eat.