The number of calories we burn while at rest varies depending on the time of day. It is linked to the role of the circadian clock in regulating metabolism.
It’s a surprising discovery: the number of calories we burn while at rest varies depending on the time of day. Men burn 10% more calories in the late afternoon than in the early morning. The study, published in Current Biology, reinforces the importance of the circadian clock in the regulation of metabolism.
It also helps explain why irregularities in eating and sleeping schedules, such as at work, can make people more likely to gain weight.
A suprise
“The idea that doing the same thing at one time or another during the day burned so many more calories surprised us,” says Kirsi-Marja Zitting, of Harvard Medical School, and director of the study. To achieve these results, his team analyzed the metabolic functioning of seven people, blurring all their external cues. For several days, their laboratory environment had no clocks, windows, phones, or internet.
“This way their internal clock oscillated at its own pace,” the scientists explained. “It allowed us to measure the metabolic rate at different times of the day.”
Regularity of habits is essential
Data showed that resting energy expenditure was lowest when the body temperature began to drop at night. Conversely, energy expenditure was greatest about 12 hours later, that is to say in the early evening. The researchers also found that the use of macronutrients varied according to the circadian phases.
We therefore have here the first characterization of a circadian profile uncoupled from the effects of activity, the sleep-wake cycle and diet. “It’s not just what we eat, but also how we rest, that influences the amount of energy we burn or store as fat,” the study said. “The regularity of eating and sleeping habits is therefore very important for general health.”
Kirsi-Marja Zitting’s team plans to further study how appetite and the body’s response to food vary depending on the time of day.
.
