Mice Robbed of Darkness Fatten Up

Time of day can affect calories' impact.

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By Janet Raloff, Science News

When it comes to weight management, the timing of dining is pivotal, a new study indicates. At least in rodents, food proved especially fattening when consumed at the wrong time of day.

As nocturnal animals, mice normally play and forage at night, often in complete darkness. With even dim chronic illumination of their nighttime environment, however, the animals’ hormonal dinner bells rang at the wrong time. Affected young adults began eating most of their chow during what should have been their rest period. The result: They fattened up and developed diminished blood-sugar control, researchers report October 11 in the Proceedings of the National Academy of Sciences.

The animals didn’t eat more or exercise less. Throughout the eight-week study, their caloric intake—and output through exercise—matched that of lean kin afforded a truly dark night.

“I suspect that what we’re doing is demonstrating that a calorie is not always just a calorie”—at least in terms of weight gain, concludes neuroscientist and team member Randy Nelson of Ohio State University in Columbus.

Although human and rodent exposure to light at night has been associated with increased cancer risks, light at night apparently was not a direct cause of the effects seen in the food intake study, notes epidemiologist Richard Stevens of the University of Connecticut Health Center in Farmington. Not an author of this study, he is a longtime investigator of light-at-night impacts.

In the new work, a dimly lit night somehow encouraged animals to down most of their food at a time when their biological clocks weren’t expecting it and when their bodies were unprepared to efficiently burn the incoming calories, Stevens says. Such data, he maintains, are “very consistent with a growing realization that our lighted environment can screw up our health in a number of different ways.”

Throughout the new study, mice received 16 hours of bright light and eight hours of either darkness or dim light. The 5 lux of illumination in the dim-night group “is like having a small portable TV in the corner of a 20-by-20-foot room,” Nelson explains. Ordinarily, the mice would eat about two-thirds of their food during their active nighttime period. But those whose night remained dimly lit instead consumed more than half of their food during the well-lit rest period.

Those animals gained more weight during the feeding trial—roughly 12 grams compared to just 8 grams in the mice getting fully dark nights. Moreover, one adipose, or fat, pad site sampled by the researchers was bigger among animals in the dim-night group. This suggests “that the increased body mass reflected increases in white adipose tissue,” the authors say.

Dim light shouldn’t have affected the animals’ wakefulness, Nelson says, since they’re used to sleeping in bright light. Somehow, an absence of darkness apparently subtly reset their biological clocks—and dinner bells.

In a second set of experiments, his team allowed some animals to eat at will; others could eat only during the light phase or only during the night phase. Animals exposed to dim light at night again grew faster and fatter, except when they were forced to eat during the night. Then they remained slim like those that had encountered eight hours of total darkness nightly.

Although 5 lux is sufficient to tinker with a rodent’s circadian rhythms—the daily cycles orchestrated by an animal’s internal clock—“it’s probably not relevant to humans,” Stevens notes. However, he points out, many people encounter much brighter light than that for hours every evening. If they continue to eat during these naturally dark periods, that food might encounter a sleepy metabolic system. Just as a low-temperature flame may fail to fully burn a log, a slow metabolic system may leave some calories unburned and then store the residual as fat.

Although light at night appeared to reset the biological clock’s time-to-dine signals, other studies have shown that changing meal times can also reset the clock, notes Gloria Hoffman, a neuroscientist at Morgan State University in Baltimore. As such, she says, chronically dining at other than the times of day the species evolved to eat—daytime for humans—may amplify light’s disruption of the biological clock.