Find out why lack of sleep may negatively affect weight loss by increasing the likelihood of weight gain through increased caloric consumption.

New research is bolstering how important sleep is for a healthy weight. When you're awake between 10 p.m. and 4 a.m., you're more likely to take in extra calories, says a study from the University of Pennsylvania. People ate an average of 553 more calories when they were kept awake until the early morning. But a lack of sleep affects more than just late-night eating. Next time you're tempted to burn the midnight oil, resist the urge. Here's why:

-Lindsay Westley

You chow down in the morning.

When you're groggy in the morning, you're more likely to eat more. Sleep-deprived participants reported being hungrier in the morning and ate larger portions at breakfast and for snacks the next day compared to participants who slept 8 hours. Researchers think that just one night of total sleep deprivation prompts you to eat more by boosting ghrelin, a hormone that stimulates your appetite.

Junk-food cravings win out.

Without enough zzz's, your brain is more likely to act impulsively and choose high-calorie foods, which means your body becomes hard-wired to crave foods like pizza and doughnuts, while healthy foods like fruits and vegetables become less appealing. According to research out of the University of California, Berkeley, the part of the brain used for complex decision-making is impaired and the part that controls desire is amplified.

Afternoon snacking gets outta control.

Lack of sleep may cause our bodies to secrete a molecule called 2-AG, which stokes hunger, according to research from the University of Chicago and the Medical College of Wisconsin in Milwaukee. Participants had higher levels of the molecule after getting 4½ hours of sleep than when allowed to sleep 8½ hours. These levels peaked in the afternoon, when snack cravings are likely to kick in.