We all know that strenuous exercise like running or swimming can help burn off extra calories. But did you know that your body is burning calories even as you sit at your desk and pore over this article? Researchers at Brigham and Women’s Hospital in Boston published a study last week describing how the time of day can influence the number of calories our body burns at rest, thanks to our internal biological clocks.  

First, let’s backtrack to how we can cut calories without even moving. If you’re sitting on the couch or lying in bed, you probably don’t feel as though you’re exerting yourself. Yet your body is still expending energy to perform basic functions to keep you alive. Physiological processes like maintaining a regular heartbeat, controlling breathing rate and regulating body temperature are all examples of how your body is hard at work, even though you may not know it. The term for the amount of energy — or number of calories — needed to maintain body functions while at rest is called the basal metabolic rate.

If we’re engaging in physical activity rather than remaining at rest, our bodies must also expend energy to maintain that level of activity, which also means that more calories are being burned. The number of calories we burn at any given time can also vary due to other factors like diet or sleep-wake cycle.

In past decades, scientists found conflicting evidence about whether the number of calories the body burned at rest remained relatively constant throughout the day, or whether this number fluctuated in accordance with our circadian rhythm — which is the body’s endogenous, internal 24-hour clock. In a study published this month, researchers at Brigham and Women’s Hospital found that the body’s resting rate of metabolism does in fact change based on our circadian rhythm.

To examine changes in metabolic rate solely based on the body’s internal clock, researchers evaluated the metabolism of seven individuals who were unaware of the actual time of day. This meant no access to clocks, phones or the internet, as well as living in a windowless room for three weeks. Though the control and experimental groups followed consistent diets and activity levels, the experimental group continuously went to bed four hours later than the previous evening. Thus, the main factor influencing their metabolic rates would be their circadian rhythm, rather than their sleep-wake cycle.

Based on the results, researchers found that the number of calories subjects burned in a day varied according to their own internal, 24-hour body clock rather than their sleep-wake cycle, activity or diet. Researchers also found specific patterns in the number of calories burned over the course of the body’s “biological day.” For example, resting energy expenditure was lowest on average at about five in the morning and highest approximately twelve hours later, at five in the evening. These timings varied according to each person, since their own internal clocks were governing metabolism and were influenced by their tendencies to be morning people or night owls.

So why might it matter that the number of calories you burn at rest varies according to your body’s circadian rhythm? The results of this study suggest that irregular sleeping and eating schedules may contribute to weight gain and obesity. For example, binging on potato chips at two in the morning — a time when your body’s resting energy expenditure might be relatively low — may mean that those calories aren’t burned off and will instead pile on.

The study also highlights the importance of maintaining a consistent eating and sleeping schedule each day. This regularity, or lack thereof, can influence our internal body clock, which can thus affect the number of calories we burn at rest at a given time.