The Science of the Midnight Snack

Why the body craves junk food late at night

The Burger King in the town where I went to college could probably more accurately be described as two Burger Kings. By day, it seemed like pretty much any other establishment of its kind, with the same hard plastic chairs and the same air that smelled, felt, and tasted like grease. But by night—well, by night, it was, as the kids say, bumpin’. My classmates and I regularly trekked through Illinois winters to wait in the line that wrapped throughout the restaurant and sometimes out the door. We called it the B.K. Lounge. There was, no kidding, a bouncer.

The point of this story is that there’s something about terrible-for-you food that makes it that much more appealing after dark. Earlier this week, Jawbone, the company behind the fitness-tracker app UP, released a trove of data on its users’ eating habits throughout the day. Among the more predictable pieces of information—people consume dairy around breakfast time and like their veggies best at dinner—is a nugget sure to comfort anyone who’s braved subzero temperatures for some midnight B.K. Cheesy Tots: You’re in good company.

People’s preference for fatty and sugary foods peaked around 12 o’clock and stayed high until just before 4 a.m.:

Food Preference by Hour

Within the category, the foods leading the pack in the later hours were ice cream, pound cake, and French fries, which makes sense: Our bodies are hard-wired for late-night junk-food cravings. Or, more scientifically, for late-night energy boosts in the form of calories—lots of them.

In a study published last year in the journal Obesity, researchers regulated the sleeping and eating habits of volunteers over a two-week stay in the lab. Regardless of when they’d woken up, when they’d last eaten, and how much they’d eaten that day, the volunteers were hungriest at night. And while their desire for vegetables stayed more or less the same throughout the day, their hunger for sweet, salty, and starchy foods increased dramatically around 8 p.m.—suggesting that our appetite for junk is governed more by our internal body clocks than by genuine hunger.

One reason for this is fluctuation in the hormone cortisol, which tells the liver to release sugar into the blood. Because we don’t need as much energy at night, cortisol levels decrease, telling our bodies that it’s time to go to sleep—when we stay awake, though, we’re driven to compensate for the resulting blood-sugar drop by eating food. But what’s bad for our modern-day waistlines may have had an evolutionary benefit, said the authors of the Obesity study: Because the body burns fewer calories at night, the urge to consume more of them in later hours may have helped our ancestors stay nourished when food was scarce.

Another reason may be that end-of-day fatigue increases the temptation to indulge. When we’re tired, we make less of the hormone leptin, which brings on a satiated feeling, and more of the hunger hormone ghrelin. As a result, our sleepy brains pull a few tricks to nudge our bodies towards energy pick-me-ups in the form of calories. In one 2012 study, researchers found that when people were sleep-deprived, the reward centers of their brains lit up more when they looked at pictures of junk foods than when they saw pictures of healthy foods (in well-rested people, the brain response was roughly the same for both food groups). And another study, also from 2012, found that lack of sleep led to reduced activity in the areas of the brain that controlled decision-making—and, as a result, to greater cravings for fattening foods over healthier ones.

In the neighborhood where I live now, the Burger King right around the corner from my apartment closed down the same week I moved in. That’s probably for the best: When late-night cravings hit, they’re usually a sign from the body that what it really needs is sleep. But if you happen to dream of deep-fried Cheesy Tots, well, no judgment here.