Unexpected discovery reveals we burn more calories in the afternoon than in the morning
A fascinating new study has revealed that our bodies burn calories at different rates throughout the day. The research, evaluating energy expenditure during resting states, suggests our circadian clock plays a major part in regulating metabolism and may help explain why people with irregular sleeping schedules or eating patterns tend to suffer from higher rates of obesity.
In order to isolate the effects of the circadian clock in relation to energy metabolism, the researchers subjected seven participants to an extensive experiment where they spent several weeks in a laboratory that removed all signs of outside time. Separated from clocks, windows, or internet, the subjects were assigned certain times to go to bed and wake up. Each consecutive night these sleep/wake times were pushed later and later.
"Because they were doing the equivalent of circling the globe every week, their body's internal clock could not keep up, and so it oscillated at its own pace," explains Jeanne Duffy, co-author on the study. "This allowed us to measure metabolic rate at all different biological times of day."
The study primarily focused on examining resting energy expenditure (REE), which studies have found to account for over 60 percent of all the calories we burn every day. The new research strikingly discovered that in a resting state our bodies burn up to 10 percent more calories in the afternoon and into the evening, compared to the early morning hours.
"The fact that doing the same thing at one time of day burned so many more calories than doing the same thing at a different time of day surprised us," says lead author on the study, Kirsi-Marja Zitting from the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School
This research is the first to reveal an explicit connection between resting energy expenditure and a person's biological clock. It follows on from other recent work that found sleep deprivation and disruption can directly alter a person's metabolic profile.
This new study is not without its limitations and, alongside the very small sample size, the researchers do note that the underlying mechanism driving these metabolic changes in relation to one's circadian rhythms is still unknown. However, there are key takeaways that can be garnered from this nascent research. The researchers suggest there isn't enough evidence to say we should immediately shift that morning workout to the afternoon or evening, but this study does affirm that the time of day we eat our meals may be vitally important.
"It is not only what we eat, but when we eat – and rest – that impacts how much energy we burn or store as fat," says Duffy. "Regularity of habits such as eating and sleeping is very important to overall health."
The next stage of the research will be to try and examine exactly how the body responds to food at different times of the day, and also finding out whether habitual behaviors can influence those responses. So don't change up your daily routine just yet, but certainly reconsider those 3 am snacks.
The new study was published in the journal Current Biology.