Night shift may increase obesity risk

People who work the night shift are more likely to be obese than those on a normal schedule because they burn less energy during a 24-hour period, a U.S. study said Monday.

Researchers have known that people who work and eat at night when their bodies are biologically prepared to sleep are prone to gain weight. But the reasons have not been clear.

The new study, published in the U.S. journal Proceedings of the National Academy of Sciences, analyzed 14 healthy adults over a six-day period.

For the first two days, the participants followed a normal schedule sleeping at night and staying awake during the day. They then transitioned to a three-day shift work schedule when their routines were reversed.

"When people are on a shift work-type schedule, their daily energy expenditure is reduced and unless they were to reduce their food intake, this by itself could lead to weight gain," said Kenneth Wright, director of University of Colorado Boulder's Sleep and Chronobiology Laboratory and senior author of the paper.

The reduction is probably linked to the mismatch between the person's activities and their circadian clocks, said Wright.