Researchers at Harvard Medical School in the US have made the surprising discovery that the number of calories that people burn while at rest depends on the time of the day. They found that when at rest, people burn 10 percent more calories in the late afternoon and early evening than in the early morning hours.
The new findings reiterate the important role that the body’s circadian clock has in governing metabolism, and could probably explain why irregularities in eating and sleeping routines due to various factors tends to make people gain weight.
To determine changes over the course of the day in metabolism apart from the effects of activity, sleep-wake cycle, and diet, the researchers studied participants in a special sealed laboratory without windows, clocks, phones or computers to indicate what time it was outside. Participants in the study had fixed times to go to bed and wake up. Each night, those times were adjusted four hours later, the equivalent of traveling westward across four time zones each day for three weeks.
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, allowing the researchers to measure metabolic rate at all different biological times of day.
The data showed that resting energy expenditure is lowest at the circadian phase the researchers designated as ~0°, corresponding to the dip in core body temperature in the late biological night. Energy expenditure was highest at circadian phase ~180°, about 12 hours later, in the biological afternoon into evening.
The researchers found that participants’ respiratory quotient, which reflects macronutrient utilization, also varied by circadian phase. This measure was lowest in the evening and highest in the biological morning.
The findings offer the first characterization of a circadian profile in fasted resting energy expenditure and fasted respiratory quotient, decoupled from effects of activity, sleep-wake cycle, and diet in humans, the researchers said.
The study showed that it is not only what we eat, but when we eat and when we rest that impacts how much energy we burn or store as fat. It also revealed the importance of regularity of habits such as eating and sleeping to overall health.
The researchers now plan to look at how appetite and the body’s response to food varies with the time of day. They are also exploring how the timing, duration, and regularity of sleep influences those responses.