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The number of calories we burn fluctuates throughout the day

A new study has revealed that the number of calories burned by an individual at rest fluctuates across different times of the day. While at rest, people burn 10 percent more calories in the late afternoon and early evening compared to the early morning hours.

The findings help explain why inconsistent eating and sleeping schedules make people more likely to gain weight.

Kirsi-Marja Zitting of the Division of Sleep and Circadian Disorders at Brigham and Women’s Hospital and Harvard Medical School is the study’s lead author.

“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 Zitting.

In an effort to examine changes over the course of the day in metabolism from the effects of activity, diet, and the sleep-wake cycle, the researchers studied seven people in a special lab with no indication of the time of day.

Study participants had assigned times to go to bed and wake up, and those times were shifted to four hours later each night. This is 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 study co-author Jeanne Duffy. “This allowed us to measure metabolic rate at all different biological times of day.”

The data showed that resting energy expenditure is lowest during the drop in core body temperature that occurs during a biological night. In addition, energy expenditure was found to be the highest about 12 hours later – in the biological afternoon.

“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. “Regularity of habits such as eating and sleeping is very important to overall health.”

Next, the team will investigate how appetite and the body’s response to food varies with the time of day. The experts are also exploring how the timing, duration, and regularity of sleep influences the responses.

The study is published in the journal Current Biology.

By Chrissy Sexton, Staff Writer

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