Nov. 9 (UPI) -- People burn 10 percent more calories in the late afternoon and early evening than in the early morning, according to a study analyzing people's rest times.
The findings, published Thursday in the journal Current Biology, show how the circadian clock, which is based on solar time, plays an important rule in regulating metabolism, according to the researchers. They believe it explains why eating and sleeping schedules due to shift in work schedules 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," lead author Dr. Kirsi-Marja Zitting, of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School, said in a Cell Press press release.
The researchers studied the changes over the course of the day in metabolism independently of the effects of activity and the sleep-wake cycle. Ten people were put in a special laboratory without any clocks, windows, phones, or Internet. Study participants were assigned when to go to bed and wake up.
Seven participants experienced three weeks of recurrent circadian disruption on a 28-hour rest-activity schedule and six three weeks on a regular 24-hour schedule at Brigham and Women's Hospital. One female participant completed the study three times and one male participant completed the study twice.
Those times were adjusted four hours later each night, 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, also in the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital. "This allowed us to measure metabolic rate at all different biological times of day."
Participants were provided controlled meals throughout the inpatient study.
They found that resting energy expenditure is lowest at the circadian phase, corresponding to the dip in core body temperature in the late biological night. Energy expenditure was highest at circadian phase 180, 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," Duffy said. "Regularity of habits such as eating and sleeping is very important to overall health."
The researchers next want to study how appetite and the body's response to food varies with the time of day. They are plans to study how timing, duration and regularity of sleep influence those responses.