Everything from your workouts to your sleep schedule can influence how many calories you burn throughout the day. And according to a small new study, there may be a time of day during which your body naturally burns the most calories.
That’s likely thanks to circadian rhythms, which control the body’s internal clock and sleep and wake cycles. These rhythms can also influence calorie burning, according to the research, which was published Thursday in Current Biology. At rest, humans burn about 10% more calories in the late afternoon than they do late at night, laboratory experiments found.
That equals about 130 extra calories burned during the late afternoon and evening versus the middle of the night, without any extra work on your part, says study co-author Dr. Jeanne Duffy, an associate professor of medicine at Harvard Medical School and a neuroscientist at Brigham and Women’s Hospital. Even a small increase like this could impact health. “If it’s happening every day,” Duffy says, “you can imagine that over time it could add up.”
Since the research focused on calorie burn at rest — that is, the energy required to power bodily functions such as breathing and blood circulation — Duffy says it’s not clear whether people should reschedule their workouts and mealtimes around this late-afternoon energy surge. What may be more relevant to day-to-day behavior, she says, is avoiding the body’s calorie-burning dip in the late night and early morning.
“Let’s say we get up an hour or two hours early and eat breakfast an hour or two hours early,” Duffy 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.”
The study only included seven people, so the findings are preliminary. But the researchers say that small sample size allowed them to conduct extensive laboratory experiments that regulated everything from people’s diets to their light exposure, offering unique insights into the natural impact of circadian rhythms.
For 37 days, the men and women in the study (who were ages 38 to 69), lived in a laboratory without clocks, windows, phones or Internet, which eliminated environmental disruptions. The researchers also carefully regulated their sleep and wake times, moving them back by four hours each day. These effects threw off the participants’ bodily clocks and forced their circadian rhythms to operate based only on internal factors, allowing the researchers to observe their body’s true biological morning, afternoon and night, separate from those on the clock. Food intake and activity levels were also defined and tracked by the researchers.
Everyone wore sensors that measured their core body temperatures, which allowed the researchers to measure energy expenditure: the higher the core temperature, the more calories the person was burning. They found that people’s body temperatures were at their lowest when circadian rhythms corresponded with late night and early morning, and at their highest about 12 hours later, in the late afternoon.
Duffy says these findings have special significance for shift and overnight workers, who often operate on unusual schedules. Research has long shown that shift work is associated with a range of health problems, including obesity, type 2 diabetes, cancer and cognitive decline. Duffy says the new study adds to the idea that these health issues may be associated with circadian rhythm disruptions.
Our biological clocks are “timed to be ready for us to do things at regular times of day and to be optimally functional. When we do things like stay up all night to work, we’re working against those internal biological clocks,” Duffy explains. “It’s not going to be optimally timed to deal with the fact that you’re eating now at 3 o’clock in the morning, when normally we don’t eat at all during the nighttime.”
More research is required to know exactly how these findings affect individuals, but the study adds to scientists’ growing understanding of the importance of circadian rhythms and their impact on total health.
“This is another metabolically related function that our bodies have that also varies with the time of day,” Duffy says. “We have these clocks inside of us that need to be synchronized and kept in sync with our external environment.”