The number of calories people burn while at rest changes with the time of day, according to new research by Harvard Medical School.
The findings reinforce the important role of the circadian clock in governing metabolism. They also help to explain why irregularities in eating and sleeping schedules due to shift work or other factors may make people more likely to gain weight.
Kirsi-Marja Zitting, the study’s lead author, said: ‘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.’
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 seven people in a special laboratory without any clues about what time it was outside. There were no clocks, windows, phones, or Internet. Study participants had assigned 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.
‘This allowed us to measure metabolic rate at all different biological times of day,’ Zitting said.
The data showed that energy expenditure was highest in the afternoon into evening.
‘It is not only what we eat, but when we eat – and rest – that impacts how much energy we burn or store as fat. Regularity of habits such as eating and sleeping is very important to overall health.’