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Scientists Have Figured Out When You Burn The Most Calories


Not all calories are equal and it turns out the time of day you tuck into dinner can affect the amount you burn – or store as fat. That's according to a preliminary study published in the journal Current Biology last week.

Researchers at Brigham and Women's Hospital discovered that when at rest, humans burn calories 10 percent faster in the late afternoon than late at night. All in all, this could add up to 130 extra calories and that's without lifting a finger. 


To find this out, the scientists recruited a small team of volunteers aged 38 to 69 and put them in a lab, where they were kept devoid of phones, the Internet, clocks, and even windows for 37 days. This meant they had absolutely no way of telling the time. Participants were also kept on a strict timetable, which involved having their food, activity, and sleep schedules carefully regulated by the researchers.

Every day, the researchers moved the day's schedule back four hours to throw the participants' internal body clocks out of whack, forcing their circadian rhythms – bodily processes that follow a 24-hour cycle – to tick along guided only by internal factors. This, the researchers say, is equivalent to traveling west through four time zones every single 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," co-author Jeanne Duffy, an associate professor of medicine at Harvard Medical School, said in a statement

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


During the experiment, the volunteers wore body temperature sensors. A high core temperature showed the person was burning more calories. A low core temperature showed the opposite. When the data came in, the researchers found that resting energy expenditure appeared to be at its lowest during the circadian phase corresponding to late nighttime and early morning and at its highest during the circadian phase corresponding to the late afternoon/early evening period, 12 hours later. 

So, what does this mean exactly?

The study looked at the rate of calorie burning at rest only, meaning it does not necessarily imply you should reschedule gym time for the late afternoon. More relevant perhaps, Duffy told Time, is the avoidance of calorie-rich foods during the slow-burning periods late at night and early in the morning – and it may also explain why shift workers are particularly susceptible to obesity and related health problems. 

"Let’s say we get up an hour or two hours early and eat breakfast an hour or two hours early," said Duffy, who also works at Brigham and Women's Hospital.


"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."

Although the research was extensive – and we certainly don't envy anyone who has had to spend more than a month cooped up in a windowless lab – it was limited to just seven volunteers and didn't explore the effects of diet, exercise, or sleep. However, this is something Duffy and her team plan to study in the future, particularly in regards to how appetite changes over the course of a day.

But in the end, as Duffy points out, "It is not only what we eat, but when we eat – and rest – that impacts how much energy we burn or store as fat."


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