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Bright dining makes mice gain weight

Study connects nighttime light exposure, body mass

When you eat might be as important as what you eat — at least, if you’re a mouse. But possibly if you’re a human too.

That’s one message from a recent study by scientists who set out to understand how obesity might be linked to exposure to light at night. According to the results of the study, when mice are exposed to dim or bright light every night, they grow fatter — and faster — than mice who get eight hours of darkness.

It’s not the light itself that’s making these mice fat. Rather, the light at night disrupts a biological process called the circadian clock. It’s not a clock with hands and dials; rather, this clock is in the brain of every mammal. Scientists believe this clock helps animals — including people — adapt to the 24-hour cycle of day and night on Earth.

This circadian clock tells the body, for example, when to sleep, when to eat and when to burn calories. The clock responds to the amount of light around the animal — so if the light changes, the clock may change. Which means a body may get mixed-up about what to eat, and when.

That’s what may have happened to the mice in the study. The mice were divided into three groups. One group was exposed to the standard amount of light and dark: 16 hours of light and eight hours of darkness. The second group lived in light for 16 hours, but spent the other eight hours in dim light.

This dim light was “like having a small portable TV in the corner of a 20-by-20-foot room,” Randy Nelson told Science News. Nelson, a neuroscientist at Ohio State University in Columbus, worked on the study. A neuroscientist studies the brain and nervous system.

The third group lived in bright light 24 hours every day.

After just one week into the experiment, the researchers noticed that the dim-light and bright-light groups were gaining weight. After eight weeks, the mice in the dim-light and bright-light groups weighed considerably more than mice in the standard group.

But here’s the surprise: The heavier mice hadn’t eaten more food — they had just eaten at different times than the lighter mice had.

Mice are naturally nocturnal. That means they’re active at night, which is when they eat the bulk of their food. In the standard group, the mice ate only one-third of their daily food intake in the light. But mice in the dim-light group ate more than half of their food during the same time — when they saw the light.

That suggests that light at night resets the body’s biological clock and sends an “eat now” signal to the brain at the wrong time. The mice may have been eating when their bodies were not prepared to digest and burn extra food, scientist Richard Stevens told Science News. This miscommunication may have caused the extra weight gain.

Just because we’re not mice doesn’t mean we’re off the hook. The researchers noted in their study that around the world, there’s a connection in human beings between obesity and increased exposure to light at night. Some people have to work the night shift; others stay up in front of their computers or televisions.

Stevens works at the University of Connecticut Health Center in Farmington and does research on the effects of light exposure at night. He was not involved with the research on mice and obesity, but says these types of studies suggest a “lighted environment can screw up our health in a number of different ways.”

POWER WORDS (adapted from materials from the NIH and the Yahoo! Kids Dictionary)

circadian clock An internal timekeeping system that allows an organism to anticipate and prepare for the changes in the physical environment associated with day and night, ensuring that the organism will “do the right thing” at the right time of the day.

nocturnal Of, relating to, or occurring in the night.

biology The science of life and of living organisms, including their structure, function, growth, origin, evolution and distribution.

neuroscience Any of the sciences, such as neuroanatomy and neurobiology, that deal with the nervous system.

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