Some of our body’s functions—including burning calories—run according to an internal schedule that has little to do with our lifestyle, a new study finds.
No matter whether we stay up all night or maintain a typical sleeping schedule, our bodies have an autopilot function that says to burn the most calories in the late afternoon and early evening, and the least in the early morning.
The study finds that people burn about 10 percent more calories from 4 p.m. to 6 p.m. regardless of the activity they’re performing—whether they’re getting off work or just waking up to start the night shift.
But the most significant discovery may concern the hours when we’re burning the least calories.
“You need fewer calories to keep going from 4 a.m. to 6 a.m.,” writes Jeanne Duffy, Ph.D., one of the study’s authors and a neuroscientist at Brigham and Women’s Hospital and an associate professor at Harvard Medical School. “But if you’re up and eating then because of a night shift or erratic schedule, then those calories aren’t being used and are instead being stored.”
Another way to put it is that people with those schedules are at higher risk of weight gain and obesity.
“The lessons we can take from it are that people like shift workers who are up all night or not eating during the days or people who are on schedules that vary a lot—and that means eating at different times, especially early in the morning or late at night, that may contribute to weight gain,” Duffy explained to a Healthline contributor.
Internal vs. external time
The body’s autopilot function, or internal clock, largely determines the rise and fall in calorie burning throughout the day. Duffy posits that our internal clock has to synchronize with the outside world every day, which occurs mainly through exposure to sunlight but also through a cycle of fasting each night and eating during the day.
“We think these kinds of rhythms have evolved because they’re beneficial to us,” Duffy says. “They allow our bodies to predict regular, occurring events and then get ready for them. So in the morning, your internal clock gets your pancreas going so it can produce the insulin that will help your body turn sugars in foods into energy it can use.”
So if you disrupt the body’s natural process of fasting during the night, skip meals, or eat breakfast at different times of the day, your internal rhythms can become less exact and less able to plan ahead, ultimately leading to weight gain.
In her research, Duffy monitored metabolism changes throughout the day among seven men and women isolated in a windowless, clockless lab over a three-week period. All food intake was carefully controlled, and all participants refrained from calorie-burning activities for the length of the study. Duffy wanted to know how our bodies use up calories while just pumping blood, breathing, and operating normally while we’re resting.
“We wanted to understand whether that (amount of calories burned) is the same no matter when you measure it or whether it varies with the time of day,” Duffy added.
They even tried to throw off the participants internal clocks by moving their sleeping and waking times back by four hours a day, all while they were sequestered in the lab. The idea was that these disturbances would compel their internal clocks to figure out the time of day themselves, without the help of sunlight or a regular eating program. In this way, the body’s calorie burning process would reveal the true biological night and day.
Duffy and her team of researchers determined that calorie burning at rest was at its lowest in the morning and at its highest in the afternoon and evening. But whether the same calorie-burning patterns would be the same if exercise were permitted—and depending on what time of day it was permitted—is still uncertain.
“The practical implications of our findings are that any irregularity in our schedules of eating and sleeping may make us more likely to gain weight,” Duffy writes.
As for how this discovery might figure into a strategy to mitigate weight gain, “keeping a very regular schedule of sleeping and waking, as well as eating, is a best practice.”
Regularity, in this case, means going to bed and getting up, as well as eating meals, at nearly the same time every day. That ensures our internal rhythms are prepared to respond optimally to the food we eat.
Since we don’t know if regular exercise would influence these findings—it’s possible that you might burn the same amount of calories while exercising no matter what time of day it is—Duffy is not satisfied.
“Maybe the energy from an early breakfast takes a long time to reach your muscles, and maybe the same exact meal in the late afternoon gives you an immediate energy boost,” she told Healthline.
But that is yet to be determined—and it’s the next question Duffy will be examining.
What does this mean?
Based on this research, it’s safe to say people who work night shifts or have rapidly changing schedules may be at greater risk of gaining weight because our internal clocks tend to use up most calories from 4 p.m. to 6 p.m. and the least from 4 a.m. to 6 a.m.
If we’re awake late and eating when our bodies are burning fewer calories, those calories get stored instead, which can add pounds rather than shed them. Still—don’t hold your breath—try to maintain as much regularity as you can, and know that exercise is good any time of the day.
If you’d like more information, the U.S. National Institute of Diabetes and Digestive and Kidney Diseases has more on weight management.