The body's metabolism at rest is regulated by circadian rhythms.
Even how many calories a person burns during rest depends on the hour. People burn about more calories, resting during the day and in the evening than in the early morning.
But morning is better for burning carbohydrates, while fats are likely to be better burned in the evening, according to researchers on November 8 at Current Biology. Conclusions suggest that the time when people eat and sleep can be just as important as what they eat to maintain proper health.
Calories that are burned at rest serve as fuel for blood circulation and brain activity, and also are spent on maintaining body temperature. Previously, researchers had conflicting data on whether a resting organism burns calories at a fairly constant speed, or whether this speed rises and falls in a daily or circadian rhythm.
As a result of the study, Jeanne Duffy from Brigham and the Women's Hospital in Boston and her colleagues showed that the metabolism at rest depends on the time of day, regulated by circadian clocks.
Researchers observed three weeks for seven people who were placed in windowless rooms. They were not given any clues about the time of day. Every night they all went to bed four hours later than the previous night. It is the equivalent of traveling the world and crossing all time zones during the week. Changing the graphics allowed the researchers to study the natural rhythms of the body of each subject without external influences.
Study participants had clear rhythms when they burned calories. But the timing of peaks and troughs can vary from person to person. For example, the best burning of calories on average occurs at about 17 hours, but some people reach a maximum at about 14 hours, and some later – at 20:00. The lowest calorie burning was on average at 5 am, but ranged from about 2 to 8 am.
This variability is normal for circadian rhythms, Duffy says. After all, some people are larks, and some are owls. The time of their daily rhythms reflects these differences.
“Regularity is really important,” says Duffy. Irregular schedules interrupt circadian rhythms, which, in turn, can reset metabolism and cause people to burn fewer calories. Studies have already shown that job changes and chronic sleep loss can lead to weight gain and health problems. No matter what time people get up or eat, it’s important that they do it on a regular schedule, even on weekends, Duffy says.