Timing is everything.even how many calories a person burns while at rest depends on the hour.

People burn about 129 more calories when resting in the afternoon and evening than in the early morning. But morning is better for burning carbohydrates, while fats are more likely to be burned in the evening, researchers report November 8 in Current Biology. The findings add to evidence that when people eat and sleep may be as important as what they eat for maintaining proper health . 

Calories burned at rest fuel breathing, circulation and brain activity, while also helping to maintain body temperature. Researchers previously had conflicting evidence about whether a resting body burns calories at a fairly constant rate, or one that rises and falls in a daily — or circadian — rhythm. The study shows that a body’s resting metabolism is governed by circadian clocks, neuroscientist Jeanne Duffy of Brigham and Women’s Hospital in Boston and colleagues report.

The study followed seven people kept in windowless rooms for three weeks, without any clues to the time of day. Each night, the seven went to bed four hours later than the previous night. That’s the equivalent of traveling around the world and crossing all time zones within a week. The schedule change allowed the researchers to study the natural body rhythms of each subject without outside influences.

Study participants all had clear rhythms for when they burned calories. But the timing of the peaks and troughs could vary from person to person. For instance, resting calorie burning peaked on average around 5 p.m., with some people peaking earlier around 2 p.m. and some later at 8 p.m. The lowest calorie burning came around 5 a.m., but ranged from about 2 a.m. to 8 a.m.

That variability is normal for circadian rhythms, Duffy says. After all, some people are morning people, and some are night owls. The timing of their daily rhythms reflects those differences.

“Regularity is really important,” Duffy says. Irregular schedules interrupt circadian rhythms, which in turn can throw off metabolism and cause people to burn fewer calories. Studies have already shown that shift work and chronic sleep loss can lead to weight gain and health problems .

It doesn’t matter exactly what time people get up or eat, but that they do it on a regular schedule, even on the weekends, Duffy says.