Some of your body’s functions run on autopilot.
That includes burning calories.
No matter whether we stay up all night or fall asleep at the dinner table, our bodies have an internal schedule that says to burn the most calories in the late afternoon and early evening and the least in the early morning.
Those are the findings of a new study, which concluded people burn about 10 percent more calories from 4 p.m. to 6 p.m. — no matter whether they’re getting off work and heading home for dinner or just waking up to start a night shift.
But the biggest takeaway may be about the hours when we’re burning the least calories.
You need fewer calories to keep going from 4 a.m. to 6 a.m., 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, explains Jeanne Duffy, PhD, one of the study’s authors and a neuroscientist at Brigham and Women’s Hospital and associate professor at Harvard Medical School in Massachusetts.
That means people with those schedules are at higher risk of weight gain.
“The lessons we can take from it are that people like shift workers who are up all night or not eating during the day or people who are on schedules that vary a lot — and that means eating at very different times, especially early in the morning or late at night, that may contribute to weight gain,” Duffy told Healthline.
Internal body clocks
The reasons for these peaks and dips in calorie burning are the body’s internal clock, which isn’t necessarily calibrated to external clocks.
Duffy explains our internal clock has to synchronize a little bit with the outside world each day.
That’s done mostly through exposure to sunlight, but also in part through a cycle of fasting each night and eating during the day.
“We think these kind of rhythms have evolved because they’re beneficial for us,” Duffy said. “They allow our bodies to predict regular, occurring events and then get ready for them.”
One of those regular events is the end of that overnight fasting period, aka breakfast.
So, in the morning, your internal clock gets your pancreas going so it can produce the insulin that will help your body turn the sugars in foods into energy it can use.
If you eat breakfast at different times of day or sometimes skip it altogether, “that can make your internal rhythms less precise and less able to anticipate,” Duffy said.
Automatically burning calories
In her research, Duffy wanted to measure how these internal clocks — or circadian rhythms — affect how many calories we use up without trying.
That is, how our bodies use up calories while just pumping blood, breathing, and humming along while we’re resting.
Duffy says these autopilot operations account for 60 to 70 percent of the energy we burn in a day.
“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 said.
So, she and her colleagues sequestered seven people in a windowless, clockless lab cut off from the outside world.
They then tried to throw off the participants’ internal clocks even more by moving their sleeping and waking times back by four hours a day.
The idea was that these disruptions would force the participants’ internal clocks to figure out the time of day themselves, without the crutches of daylight or regular bedtimes or mealtimes.
That would reveal the true biological night and day.
With sensors, they measured body temperatures and from that deduced the number of calories being burned. They found that temperatures were lowest during the deep biological night and highest about 12 hours later.
But can you make your own biological night or late afternoon occur at a different time if, say, you’ve been working a night shift for years?
Duffy is skeptical that would be possible.
Even people who are late-shift workers often operate — or try to — on a more normal schedule on days off, she says.
This prevents internal clocks from getting permanently changed.
Plus, going home in the morning after working all night means you’re likely exposed to sunlight — the biggest signal for our internal clocks that they should reset to start the biological day.
It’s just built in.
Timing your exercise
For people on evening or overnight schedules, this all could mean weight gain.
Does it work the other way, too?
Do these findings mean that it would be best to work out in the late afternoon and early evening, when our body is already primed to burn a bunch of calories?
We don’t know yet, Duffy says.
She and her colleagues studied resting energy expenditure.
Exercise energy expenditure might follow the same patterns or it might be independent of these circadian rhythms.
It’s possible, she says, that you might burn the same amount of calories while exercising no matter what time of day it is — or what time your body thinks it is.
It’s also possible your body might take longer to process a meal at different times of day.
Maybe the energy from an early breakfast takes a long time to reach your muscles, and the same exact meal in the late afternoon gives you an immediate energy boost.
Or maybe not.
That’s the next question Duffy and her team are examining.
The bottom line
People who work night shifts or have constantly changing schedules may be at greater risk of weight gain, according to a new study.
That’s because our internal clocks tend to use up the most calories from 4 p.m. to 6 p.m. and the least from 4 a.m. to 6 a.m., researchers concluded.
If we’re up late and eating when our bodies are using up fewer calories, those calories get stored instead, which can result in weight gain.