The Number of Calories You Burn Depends on the Time of Day, Say Scientists

People hate on daylight savings time because it not only screws with the time on our stoves or car dashboards (why don’t they automatically update?) but also the internal clocks in our bodies. It doesn’t just mess up your sleep schedule by one weird hour; as researchers at Brigham and Women’s Hospital in Boston show in a new paper, it could also have powerful effects on how efficiently a body burns calories.

Obviously, we tend to burn more calories both during and after activities, but the body also burns calories just to keep things functioning during rest. This is called “resting energy expenditure,” or REE. Previous work has suggested that REE tends to fluctuate according to the time of day in older adults. The new findings put forth in Current Biology by the Boston researchers present more detailed evidence supporting the idea. During “biological” afternoon and early evening, they write, the body burns calories at a ten percent higher rate than it does during biological morning.

Study co-author and associate professor at Harvard Medical School Jeanne Duffy, Ph.D., tells Inverse that this cycle of calorie burning corresponds to the concept of a “biological day,” which takes its cues from the environment — namely, sunlight. As a result, it can become easily confused by temporal factors like strange schedules, traveling into new time zones, or the one day each fall when the world decides to adopt slightly different waking hours.

“Biological day and night refer to the time within someone’s body,” Duffy says. “For example, if you fly from New York to Bangkok, they are 12h apart. When you arrive in Bangkok it is noon, but inside your body it is midnight. So even though the clock says it is noon, it is your biological night time.”

Duffy tried to eliminate as many of the external cues that dictate this rhythm as possible, focusing purely on the body’s internal state of calorie burning to show how powerful this internal clock really is. The end result was a very involved experiment that could not have been easy on its participants.

For 37 days, she kept seven people in a room devoid of windows, doors, or any other external cues identifying the time of day. She also gave them a specific schedule of wakeup times and bedtimes. Over the course of the experiment, Duffy shifted the schedule by pushing bedtimes back four hours later over. By doing this, she was able to wean her participants off of their natural circadian rhythms shaped by outside factors, thus allowing their confused bodies to dictate their own cycles — which she calls an endogenous circadian rhythm.

In both this group and the control group, which got to keep its natural circadian rhythm, the body burned the least amount of calories at rest during biological night — when the body’s core temperature (one way to determine cycles of energy expenditure) reaches its lowest point. In the twelve hours after biological night, the body slowly increases its rate of calorie burning, peaking around biological afternoon or early evening.

People with endogenous circadian rhythms showed the highest rates of calorie burning around 5 am and the lowest around 5 pm, though these numbers, she points out, are arbitrary since she was manipulating their body clocks, they could have been any time. What this shows is that calorie burning at rest reaches a peak 12 hours after its lowest point — what hour of the day those peaks correspond to are dependent on the schedule a person chooses to keep.

Whether you’re a night owl or early riser, Duffy suggests you keep a regular schedule. Previous research has shown that people run into issues like increased obesity when patterns of sleeping and waking differ wildly from day to day.

“The implications of this, together with other recent finding are that keeping a very regular sleep/wake and fasting/feeding schedule will be best for overall health and specifically for metabolic health,” Duffy adds.