Researchers link time of the day to number of calories burnt

The number of calories people burn changes with the time of day, according to a study which could explain why irregularities in eating and sleeping may make people gain weight.

Researchers from Brigham and Women's Hospital and Harvard Medical School in the US found that when at rest, people burn 10 per cent more calories in the late afternoon and early evening than in the early morning hours.

The findings, published in the journal Current Biology, reinforce the important role of the circadian clock in governing metabolism.

The study could also help explain why irregularities in eating and sleeping schedules due to shifting work or other factors 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 Kirsi-Marja Zitting from the Brigham and Women's Hospital and Harvard Medical School.

To determine changes over the course of the day in metabolism apart from the effects of activity, sleep-wake cycle, and diet, the researchers studied seven people in a special laboratory without any clues about what time it was outside.

There were no clocks, windows, phones, or internet.

Study participants had assigned times to go to bed and wake up. Each night, those times were adjusted four hours later, the equivalent of travelling 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 Jeanne Duffy from Brigham and Women's Hospital.

"This allowed us to measure the metabolic rate at all different biological times of day," Duffy said.

The researchers found that participants' respiratory quotient, which reflects macronutrient utilisation, varies by circadian phase, too.

This measure was lowest in the evening and highest in the biological morning, researchers said.

The findings offer the first characterisation of a circadian profile in fasted resting energy expenditure and fasted respiratory quotient, decoupled from effects of activity, sleep-wake cycle, and diet in humans, they said.

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

"Regularity of habits such as eating and sleeping is very important to overall health," he said.

(With inputs from agencies.)