By Shawn Radcliffe

A new study found that sleep deprivation, coupled with a late bedtime, was linked to weight gain and more late-night snacking, especially in men.

If you’re reading this in the middle of the night, put down that slice of pizza and head to bed. Your party-all-night lifestyle could be leading you down the road toward unhealthy weight gain.

A new study, one of the largest of its kind, found that healthy adults who slept only four hours a night and went to bed at 4 am gained more weight than their well-rested counterparts—after just five days of sleep deprivation.

The weight gain, which amounted to 2.1 pounds on average, was largely due to people eating more food late at night (yes, all those quick trips to the convenience store add up). In particular, men were more affected by the lack of sleep, gaining up to three times as much weight as women.

The new study, which was published in the July issue of the journal Sleep, also found that people shifted toward eating fattier foods after several days of less-than-optimal sleep.

Previous studies, however, have shown more immediate sleep-related dietary shifts—along with an increased intake of carbohydrate-rich foods. This may indicate that eating habits can sometimes change slowly as the effect of sleep deprivation builds up, a condition which many in our always-on-the-go culture are familiar with.

Lack of Sleep Affects Long-Term Health

In 2009, 35.3 percent of adults in the U.S. reported sleeping less than 7 hours a night, according to the Centers for Disease Control and Prevention. This is less than the recommended 7 to 9 hours of sleep that most adults need.

Burning the midnight oil (or the post-midnight oil) may give you more time for work and social activities, but the effects of sleep deprivation add up.

Long-term sleep deprivation can increase the risk of developing diseases like high blood pressure, diabetes, depression, obesity and cancer. Excessive tiredness during the day can also lead to injuries on the job or accidents while driving.

Weight gain, in particular, is a deadly combination with lack of sleep. Obese people are more likely to suffer from obstructive sleep apnea, a condition which causes people to stop breathing periodically while asleep.

Study Reinforces Importance of Sleep

The current study, by University of Pennsylvania researchers, included 225 healthy, non-obese people between 22 and 50 years old. Participants spent up to 18 consecutive days in a sleep laboratory, and were assigned to either several days of normal sleep—between 10 and 12 hours in bed each night—or late-night sleep deprivation.

For five days, people in the late-night group were only allowed to sleep for four hours between 4 am and 8 am. This was preceded and followed by two nights of normal sleep.

Researchers carefully controlled the light in the room to match sleep times—light has been shown to affect the quality of sleep. They also monitored what and how much food people ate throughout the study.

The results reinforced the importance of adequate sleep as part of a healthy lifestyle.

“Although previous epidemiological studies have suggested an association between short sleep duration and weight gain/obesity, we were surprised to observe significant weight gain during an in-laboratory study,” lead author Andrea Spaeth, a doctoral candidate in the psychology department at the University of Pennsylvania in Philadelphia, Pa., said in a press release.

The study, however, included only healthy people, so the effect of sleep deprivation may not be the same for people with illnesses or those who are already obese.

Also, sleep deprivation may not bother people as much if they go to bed earlier. The weight gain seen in this study was largely due to late-night snacking, so people who sleep at other times—even with only 4 hours in bed each night—may eat less food. More research is needed to answer these questions.

Based on the research that’s been done so far, though, there is no doubt that sleep deprivation has its downsides when it comes to your health (besides the dark circles under your eyes).

So if you tend to squeeze more into your life at the expense of solid shut-eye, you may want to reconsider before all those late-night runs for pizza and junk food start to add up … on your waist.