Researchers have made the surprising discovery that the number of calories people burn at rest changes over the course of the day. At rest, people burn 10 percent more calories in the late afternoon and early evening than in the early morning. The findings reinforce the important role of the circadian clock in regulatory metabolism. They also help explain why irregularities in eating and sleeping schedules due to shift work or other factors can cause people to gain weight more often.
To determine the changes over the day in metabolism, aside from the effects of activity, sleep-wake cycle and diet, the researchers studied seven people in a special lab with no clues about what time it was outside. There were no clocks, windows, telephones or internet. Study participants were assigned times to go to bed and wake up. Each night, those times were adjusted four hours later, the equivalent of traveling west across four time zones every day for four weeks.
The data showed that resting energy expenditure is lowest in the circadian phase which the researchers designated as ~0°, which corresponds to the drop in the body's core temperature in the late biological night. Energy expenditure was highest in the circadian phase ~180°, about 12 hours later, in the biological afternoon to evening.
The researchers found that the participants' respiratory quotient, which reflects macronutrient utilization, also varies with circadian phase. This measure was lowest in the evening and highest in the biological morning.
The findings provide the first characterization of a circadian profile in rapid resting energy expenditure and rapid breathing quotient, decoupled from effects of activity, sleep-wake cycle and diet in humans, the researchers say.
"It's not just what we eat, but when we eat — and rest — that influences how much energy we burn or store as fat," said the researcher. “Regularity of habits such as eating and sleeping is very important for overall health.”
