People at rest burn more energy in afternoon than in morning: study

A study published on Thursday in the journal Current Biology found that when at rest, people burn 10 percent more calories in the late afternoon and early evening than in the early morning hours.

It helped to explain why irregular eating and sleeping due to shift work may make people more likely to gain weight.

"The fact that doing the same thing at one time of day burned so many more calories than doing the same thing at a different time of day surprised us," said lead author of the paper Kirsi-Marja Zitting of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School.

Participants in the study were put in a special lab without any clues about what time, but with assigned times to go to bed and wake up. Each night, those times were adjusted four hours later, the equivalent of traveling westward across four time zones each day for three weeks.

"Because they were doing the equivalent of circling the globe every week, their body's internal clock could not keep up, and so it oscillated at its own pace," said co-author Jeanne Duffy at Brigham and Women's Hospital. "This allowed us to measure metabolic rate at all different biological times of day."

The findings showed that resting energy expenditure is lowest at the circadian phase in the late biological night and highest about 12 hours later, in the biological afternoon into evening.

"It is not only what we eat, but when we eat and rest that impacts how much energy we burn or store as fat," said Duffy.