Even when you're not really doing anything, you're burning calories. But how many you burn depends on the time of day, researchers found in a new small study.
The study found that, at rest, people burn 10 percent more calories in the late afternoon and early evening than in the early morning.
It's likely because of circadian rhythms, which control our sleep-wake cycles and our internal body clock.
"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 Kirsi-Marja Zitting of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School, in a statement.
The study was published in the journal Current Biology.
The study only focused on calories burned while at rest, which is the energy your body needs to run basic bodily functions like breathing and circulating blood throughout the body. So researchers aren't sure if people would benefit from planning their exercise in the late afternoon and early evening.
Study co-author Dr. Jeanne Duffy, an associate professor of medicine at Harvard Medical School and a neuroscientist at Brigham and Women's Hospital, tells Time that it may be more relevant to avoid the body's calorie-burning dip in the late night and early morning.
"Let's say we get up an hour or two hours early and eat breakfast an hour or two hours early," Duffy says. "We may be eating that breakfast not only at a time when our body might not be prepared to deal with it, but at a time when we need less energy to maintain our functions. Therefore, the same breakfast might result in extra stored calories, because we don't need those to maintain our body functions."
The closed-door study
To follow the impact on body clock and calories, the researchers had seven volunteers spend a month in a lab that had no windows, clocks, phones or internet. They were assigned times to go to bed and wake up, with those times changing four hours each night. They kept this schedule 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," explained Duffy. "This allowed us to measure metabolic rate at all different biological times of day."
The study found that people's body temperatures were at their lowest when their body clock was set to late night, and at their highest about 12 hours later, in the late afternoon.
The findings may help explain why people who don't have regular schedules, like night-shift workers, are more likely to gain weight, the researchers said.
People who want to lose or maintain weight should stick to regular sleep and eating habits, they suggest.
"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 says. "Regularity of habits such as eating and sleeping is very important to overall health."