The later you stay up, the more you eat, study shows

A man eats potato chips while watching TV.
Dude, it's time to go to bed. A new study found people who got only four hours of sleep for five consecutive nights ate more fatty foods and gained 2.2. pounds on average.Ryan McVay / Today

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By Brian Alexander

New research published today in the journal Sleep confirms what scientists have been telling us for some time now: the later you stay up, the more sleep you lose, the more calories you’ll eat, and the more weight you’ll gain.

In a first-of-its-kind experiment, sleep researchers from the Sleep and Chronobiology Laboratory at the Perelman School of Medicine at University of Pennsylvania, found that when test subjects went to bed at 4 a.m. and woke four hours later at 8 a.m., they consumed about 550 calories after 11 p.m., far more than their bodies needed. In addition, more of those 550 calories came from fat, resulting in a weight gain, on average, of about 1 kilo (2.2. pounds) after five consecutive nights of limited sleep.

Research over the years has produced similar findings, but the new Penn study is significant because it used 225 people. And because the research took place in a lab, scientists were able to control for a variety of factors.

Andrea Spaeth, a doctoral candidate and the study’s lead author, called the logistics “a nightmare” but worthwhile because it increased the power of the results.

Spaeth said participants in the study could eat almost whenever they wanted and as much as they wanted. Meal sizes did not differ between baseline days and the five experimental days, or between the sleep-deprived group and a small control group. “But [the deprived group] ate more times through the day and shifted their calorie intake toward late night,” she explained. “The next morning they’d actually eat less.”

African-Americans were especially vulnerable to the effects of sleep deprivation. African-American men gained the most weight (about 1.7 kilos or 3.7 pounds), followed by African-American females and white men (both just under 1 kilo or 2.2 pounds), and white women (almost one-third kilo or .7 pounds).

“This is the first racial difference in such a study,” explained Kenneth Wright, associate professor in the department of integrative biology and the director of the Sleep and Chronobiology Laboratory at the University of Colorado, Boulder. “This provides experimental evidence for what we see in the epidemiological literature, that African-Americans seem to be more influenced by the effects of sleep loss.”

Why that is, and why exactly anybody gains weight with sleep loss is still somewhat mysterious, Wright explained. In an experiment released earlier this year, his lab found that key hormones regulating food intake, like ghrelin and leptin, were within normal levels in sleep deprived people, but they still ate more.

It could be, he speculated, that control centers of the brain – i.e., willpower – are weakened, causing us to seek rewarding, pleasurable food, which could explain why we tend to eat fattier food late at night. We’re seeking comfort. But there are other factors at work, too.

“We have biological mechanisms to prompt us to consume more energy” when we’re awake, Spaeth said. But such evolutionary adaptation does not account for the fact that modern man in western society no longer has to hunt down an antelope or dig up a tuber to get food. All we have to do is open the refrigerator. “What we’re doing is really overcompensating for small increases in energy requirements.”

This is partly why shift work can be so detrimental to health, Wright said. “Shift work goes against the fundamental biological clock in our brain. We evolved to be awake during the day when we are supposed to be physically active and consuming food. When we are awake at night, the circadian clock does not adapt. There are consequences to that…That’s why shift workers are at greater risk for many health problems we see in modern society.”

Shift work isn’t going away any time soon, though. Wright said finding ways to compensate for this circadian havoc is a key goal of chronobiological research.

Brian Alexander ( is co-author, with Larry Young Ph.D., of "The Chemistry Between Us: Love, Sex and the Science of Attraction," (, now on sale.