Your body’s internal clock, or circadian rhythm, dictates a lot of your day. It tells you when you’re hungry and need to scarf some food, when you’re tired and should hit the hay, and more. That body clock also determines what time of day your body burns the most calories, even when you're at rest doing nothing, a new study shows.
Researchers reporting in Current Biology found that when resting, we burn 10% more calories in the late afternoon and early evening than in the morning. So that means we don’t have to worry about being lazy next time we hit an afternoon slump, right? Our bodies will pick up the slack for us.
This study’s findings reinforce the important role circadian rhythm plays in regulating metabolism. They also shed some light on why those who have irregular sleep schedules because they work night shifts or other factors are more likely to gain weight.
To evaluate changes in metabolism throughout the day without the effects of activity level, dietary habits, and sleep patterns, the researchers studied seven participants for over a month in a laboratory that had no windows or clocks. Participants didn’t have access to phones or internet, and they were given schedules of when they could sleep, wake up, and eat.
We know what you’re thinking. No windows? No phones? Were they kidnapped? Calm down, we promise they did this voluntarily.
Each night, the participants went to sleep four hours later than the night before. This mimicked what a person would experience when traveling westward across the entire world in a week.
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"Because they were doing the equivalent of circling the globe every week, their body's internal clock could not keep up," co-author Jeanne Duffy, of the division of sleep and circadian disorders at Brigham and Women's Hospital in Boston, said in a statement. That left it up to the body to keep its own time without relying on cues from the outside world. "This allowed us to measure metabolic rate at all different biological times of day."
Researchers found the participants burned the fewest resting calories late in the biological night, when people experience a drop in their body temperature. Energy expenditure was highest about 12 hours later, in the biological afternoon and evening.
The study itself was small, but the results help shed light on how circadian rhythm influences metabolism. Going forward, the researchers suggest that future studies examine whether these changes in resting metabolic rate contribute to weight gain among people who don’t keep regular sleep schedules. Until then, anyone who’s trying to lose weight should maintain a normal schedule, which is important for overall health.
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