Misalignment of the biological clock may alter calories burnt, leading to obesity risk among those with irregular schedules
How many calories you burn depends on time of day, finds a new study.
Researchers have made the discovery that the number of calories people burn while at rest changes with the time of day. When at rest, people burn 10 percent more calories in the late afternoon and early evening than in the early morning hours.
The findings may help to explain why irregularities in eating and sleeping schedules due to shift 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 of the Division of Sleep and Circadian Disorders at Brigham and Women’s Hospital and Harvard Medical School, lead author of the paper.
When at rest, people burn 10 percent more calories in the late afternoon and early evening than in the early morning hours
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 traveling westward across four time zones each day for three weeks.
The data showed that resting energy expenditure is lowest corresponding to the dip in core body temperature in the late biological night. Energy expenditure was highest at about 12 hours later, in the biological afternoon into evening.
The findings offer the first characterization of a circadian profile in fasted resting energy expenditure decoupled from effects of activity, sleep-wake cycle, and diet in humans, the researchers 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,” said co-author Duffy. “Regularity of habits such as eating and sleeping is very important to overall health.”
Findings of the study were published in  Current Biology.
