Maybe you’d prefer to come back around dinner time.
This pattern of calorie use doesn’t significantly vary based on whether you’re the waitress 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 new study, published this week in the journal Current Biology, offers further 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 am – our core body temperature dips to its lowest point and our idling fuel use reaches its nadir. From that point, at first quickly and then a bit more slowly, the body’s “resting energy expenditure” rises until the late afternoon/early evening. After reaching its peak at roughly 5pm, the number of calories we burn while at rest plummets steadily for about 12 hours.
And then, just as surely as day follows night, we start again.
These new findings are a reminder that no matter how 24/7 our schedules have become, our bodies were built for a slower, simpler world in which humans moved around all day in search of food, ate while the sun was up, and slept when the sky was dark.
Today, our appetites and the all-night availability of tempting food may induce us to eat well after sundown. And our jobs may demand that we sleep during the day and wait tables, care for patients or drive trucks through the night. But our bodies still adhere to their ancient, inflexible clocks.