How our bodies clock calories

Next time you stagger into a Waffle House in the wee hours of the morning and order the Texas sausage egg & cheese melt (1,040 calories), consider this new research finding: At roughly that hour, the most basic operations of the human body throttle back their caloric needs by about 10 percent compared to the rate at which they will burn calories in late afternoon or early evening.

Maybe you'd prefer to come back around dinnertime.

This pattern of calorie use doesn't significantly vary based on whether you're the server working the graveyard shift or a 9-to-5'er stopping in for breakfast after eight hours of shut-eye, the researchers found. Humans' "resting energy expenditure" — the body's use of calories to power such basic functions as respiration, brain activity and fluid circulation — follows a predictable cycle that waxes as the day progresses and wanes as night sets in.

The study, published in the journal Current Biology, offers evidence that circadian rhythms dictate not just when we feel the urge to sleep but how complex mechanisms like metabolism operate across a 24-hour period. It may help explain why people who keep irregular sleep schedules, including swing shift workers, have higher rates of obesity and are more likely to develop metabolic abnormalities such as type 2 diabetes.

And it demonstrates that whether we hear it or not, our body's clock is always ticking, locating us in our daily cycle with uncanny precision.

At "hour zero" — roughly corresponding to somewhere between 4 and 5 a.m. — our core body temperature dips to its lowest point and our idling fuel use reaches its nadir. From that point, the body's "resting energy expenditure" rises until the late afternoon/early evening. After reaching its peak at roughly 5 p.m., the number of calories we burn while at rest plummets steadily for about 12 hours.

And then, we start again.

Today, our appetites and the all-night availability of food may induce us to eat well after sundown. And our jobs may demand that we sleep during the day and care for patients or drive trucks through the night. But our bodies still adhere to their ancient, inflexible clocks.

The findings also come with an implicit warning: When we disregard the biological rhythms that rule our bodies, we do so at our peril.

Resting energy expenditure accounts for the majority of the minimum calories we burn in a day. Just to spend a day eating, sleeping and breathing uses 60 to 70 percent of our "resting energy expenditure." So a serious mismatch in the time when calories are consumed and the time when most of them are burned could prompt the body to make decisions — like storing calories as fat — that aren't necessarily healthy.

The study adds to a growing body of evidence suggesting that a good 12-hour fast, when aligned with darkness and our bodies' nocturnal response, may be a way to prevent or reverse obesity. In lab animals and a growing number of people, Salk Institute researcher Satchin Panda has demonstrated the impact of dietary obedience to our circadian rhythms.

Others have demonstrated the power of timing by showing how readily it can be disrupted. In a 2014 study, 14 lean, healthy adults agreed to turn their days upside-down over a six-day period. Fed a diet sufficient to maintain their weight, the subjects quickly adapted by turning their thermostats down. Compared to the baseline readings taken upon their arrival (when they were awake by day and asleep eight hours at night), the subjects burned 52 fewer calories on day 2 of their swing-shift schedule, and 59 fewer calories on day 3 of that schedule.

"One takeaway is indeed that for optimal health, including metabolic health, it's best for us to have a regular schedule seven days a week — getting up and going to bed at the same time and eating our meals at the same time," said senior author Jeanne F. Duffy, a neuroscientist and sleep specialist at Brigham & Women's Hospital in Boston. "We have these powerful clocks in ourselves."