While lounging around, human bodies burn about 10 percent more calories in the late afternoon than they do in the early morning, a small study finds. The results, which appear today (November 11) in Current Biology, suggest the effect is independent of external cues such as light, but instead depends entirely on internal circadian rhythms.
“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,” says coauthor Kirsi-Marja Zitting of Brigham and Women's Hospital and Harvard Medical School in a statement.
Science News notes that there’s been conflicting evidence about whether human bodies at rest burn calories at a constant rate, or whether the rate varies with time of day. To try to resolve the question, Zitting and her colleagues put each of seven volunteers in an individual room without windows, clocks, or internet access for 37 days. The subjects’ prescribed bedtime was pushed back by four hours per day, and they ate a controlled diet. “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,” coauthor Jeanne Duffy of Brigham and Women’s explains in the press release. “This allowed us to measure metabolic rate at all different biological times of day.” Those biological times were determined by tracking participants’ core body temperatures.
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The researchers found that on average, inactive subjects burned the fewest calories at a time corresponding to a biological time of about 5 am, and the most calories 12 hours later.
Duffy tells Time that a takeaway of the study is that keeping to a regular schedule is important for health. “Let’s say we get up an hour or two hours early and eat breakfast an hour or two hours early,” she 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.”