The new findings may not help you lose weight, but they reinforce the importance of maintaining a regular sleep, burn most calories and eating schedule.
Weight loss would be so much easier if we had a magic formula telling us the best time of day to work out to burn the most calories, wouldn’t it? Unfortunately, that’s not yet possible, but new research helps scientists understand how it might work.
A small, preliminary study published in November 2018 in the journal Current Biology suggested the body’s resting metabolism (how many calories you burn at rest) changes throughout the day, peaking during the afternoon and evening, and dipping during the late night. The finding surprised researchers because it showed that timing matters when measuring an individual’s resting metabolism, which is also known as resting energy expenditure (REE).
“The conventional wisdom is that if you want to measure someone’s REE, you don’t really pay attention to what time of day you do it. You take the measurement and assume that it’s the true measurement,” says senior study author Jeanne Duffy, PhD, associate neuroscientist in the division of sleep and circadian disorders at Brigham and Women’s Hospital in Boston.
The findings don’t necessarily show if weight loss is easier at different times of day, Dr. Duffy says, because the team measured resting energy that the body uses for basic functions, like breathing and blinking, not expended energy, like when you’re running or biking.
“It’s really a leap of logic to say that we should be thinking that we might burn more or less calories at different times of day,” Duffy says. “We don’t know whether you expend the same amount of calories at various times of day with activity.”
Why the New Findings May Matter for Shift Workers
Although the study results may not help you maximize your weight-loss efforts yet, the takeaway may be useful for people who do shift work or have irregular sleep schedules.
In the research, 10 participants were assigned a normal 24-hour sleep schedule or a variable sleep schedule for three weeks. The latter group’s sleep schedule increased by four hours every night, so that within a week it was as though they’d traveled completely around the globe. Authors conducted the study in a room without time cues, meaning no windows, clocks, phones, or internet access.
Humans’ circadian rhythm has evolved according to the earth’s 24-hour cycle, and has difficulty adapting to rhythms that are shorter or longer. Think of it this way: When you travel across time zones, you get jet lag because your clock has to try to adapt to those time shifts.
“For a shift worker, shifting their circadian rhythm is difficult because they have to adapt quickly — working all night and sleeping during the day — and repeatedly going back to a regular sleep schedule only to have their work shift change again,” says Jonathan Cedernaes, MD, PhD, research associate in the department of endocrinology at Northwestern University Feinberg School of Medicine in Chicago. “Like traveling across time zones, the body isn’t good at adapting.” Dr. Cedernaes was not involved in the current study.
Using sensors to measure participants’ core body temperatures, researchers found that at rest their bodies burned about 10 percent (about 130 calories) more in the late afternoon than they did at night. That might not sound like a lot, but in the long term, it might matter for those who are chronically disrupting their circadian rhythm, like a shift worker or someone who travels across time zones frequently, Cedernaes says.
Researchers plan to continue analyzing the findings to develop strategies for shift workers to regulate their metabolism and manage their weight.
“We want to be able to design patterns of eating so that people can still do shift work and reduce the negative impact that work has on their weight and other aspects of their metabolic health,” Duffy says.
Also worth noting is past research also suggests shift work can pose health harms. Per the National Institutes of Health, it can affect metabolism, and a study published in February 2016 in the Journal of Circadian Rhythms showed it could impede work performance. Meanwhile, another study published in April 2016 in the journal JAMA found that shift work was associated with a higher risk of heart disease in women.
Why Having a Sleep Schedule Is Good for Weight Management
The most important message from her team’s study, Duffy says, is to keep a regular sleep and eating schedule from day to day, so your body’s biological clock can keep your metabolism in sync.
“The crucial thing is going to bed and getting up at almost the same time of day,” Duffy says. “And to be eating meals, especially breakfast, at almost the same time every day within a half hour to hour range.”
Eating meals when your body isn’t optimally prepared to do so may lead you to overeat and take in more calories than you need, causing your body to store them.
“If you kept a very irregular schedule, it could lead to weight gain,” Duffy says.