We now know when we burn the most calories, thanks to 10 volunteers in a brutal study

No matter what time we keep externally, our bodies tick to their own clock.
No matter what time we keep externally, our bodies tick to their own clock.
Image: Reuters/Jean-Paul Pelissier
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Contemporary science can explain many of our bodily processes, but has struggled to understand the variation of our metabolisms, or how we burn calories, throughout the day. One problem is that it’s exceedingly difficult to isolate the impact of our natural circadian rhythms to those of “clock time,” the socially constructed patterns of the day. So, a group of scientists did what scientists do: created a totally insane experiment to isolate the variables.

In fact, in a new study, the scientists—a group from Harvard Medical School, Brigham and Women’s Hospital, and Penn State University—literally isolated their subjects from the world. For 37 days, the 10 men and women who agreed to participate in the study lived in a lab with no windows, no clocks, no phones, and no internet. Presumably, they had a lot of reading material.

The results offered evidence that the body’s resting metabolism fluctuates according to our circadian rhythms: the internal cycle of hormones that less tell us when we should be getting ready for bed and when to wake up, among other things. The participants’ internal temperatures were the highest, meaning they were burning the most calories, during their bodies’ biological afternoon, and burned the fewest in the biological night. At their fastest, bodies burn about 130 more calories than they do at their slowest.

The researchers worked with 10 healthy volunteers, five men and five women between the ages of 38 and 69. (All of the women were postmenopausal. That was purposeful; menstrual cycles can also affect the circadian rhythm (paywall)). Some volunteers repeated the experiment, so in the end the researchers had a total of 13 separate trials.

In six trials, participants went to bed and woke up regularly, getting eight to 10 hours of sleep at night on a regular schedule. But in the other seven trials, researchers extended participants’ days by about four hours—similar to flying to a time zone behind your own. “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,” Jeanne Duffy, a neurologist studying sleep and circadian rhythms at Harvard Medical School and lead author of the paper, said in a statement.

Researchers measured their core body temperatures rectally (another less-than-fun aspect of the experiment). Higher body temperatures mean more calories are being burned. Over time, the seven volunteers in the experimental group had the lowest internal temperatures around the “night” of their circadian rhythms. If they were keeping a 24-hour schedule, that would correspond with the middle of the night and early morning. They burned the most calories 12 hours later, which is usually around the afternoons and early evenings. If you were to picture these fluctuations like a wave, at the peaks they were burning about 55.2 calories more per hour than at the nadirs, meaning that bodies at rest burned about 10% more in the afternoons and in the evenings than they do in the early mornings.

Duffy explained to Time that our bodies work best when our sleeping and eating schedules are consistent. “When we do things like stay up all night to work, we’re working against those internal biological clocks,” she said. Eating late at night (or very early in the morning) could contribute to weight gain because our bodies aren’t burning as many calories at that point as they do at other parts of the 24-hour daily cycle.

There’s a lot more work to be done to explain the relationship between circadian rhythms and metabolism. This study was small, and only looking at resting metabolisms—the energy we need to be awake, breathe, and pump blood. But it does pave the way for future work to look at why those with irregular sleeping habits, like shift workers or other people prone to all-nighters, may be more likely to gain unwanted weight.