Nighttime eating—even the healthy kind—may cause weight gain and contribute to other concerning health risks.

By Julia Malacoff
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Photo: Joshua Resnick/Shutterstock

You've probably heard that it's bad to eat late at night if you want to lose weight. That means regular late-night pizza slices and ice cream runs are no-nos. (Bummer!) On the flip side, you may have also heard that eating late at night may help you burn calories and that it's fine to eat before bed, as long as it's a healthy snack that's on the smaller side with the right macronutrients (protein and carbs!). So, which is it? A new, yet-to-be-published study presented at the annual Sleep Meeting may answer that question. (Related: Will Eating Late at Night Make You Fat?)

For the first eight weeks of the study, people were allowed to eat three meals and two snacks between 8 a.m. and 7 p.m. Then, for another eight weeks, they were allowed to eat the same amount between noon and 11 p.m. Before and after each eight-week trial, the researchers tested everyone's weight, metabolic health (blood sugar, cholesterol, and triglyceride levels) and hormonal health.

Now the bad news for night-eaters: People gained weight and experienced other negative metabolic and hormonal changes when they ate later.

In terms of hormones, there are two main ones the authors focused on: ghrelin, which stimulates appetite, and leptin, which helps you feel satiated after eating. They found that when people were mainly eating during the day, ghrelin peaked earlier in the day, while leptin peaked later, meaning that the daytime eating schedule potentially prevented overeating by helping people feel fuller toward the end of the day, and thus less likely to indulge at nighttime.

Understandably, this is a little confusing given the previous research, but the authors of the study are pretty clear that these results mean that late-night eating is something people should probably stay away from. "While lifestyle change is never easy, these findings suggest that eating earlier in the day may be worth the effort to help prevent these detrimental chronic health effects," said Kelly Allison, Ph.D., in a press release. Allison, senior author on the study, is an associate professor of psychology in psychiatry and director of the Center for Weight and Eating Disorders at Penn Medicine. "We have an extensive knowledge of how overeating affects health and body weight," she said, "but now we have a better understanding of how our body processes foods at different times of day over a long period of time."

So what's the bottom line here? Well, past research does indicate that a late-night snack that's no more than 150 calories and mostly protein and carbs (like a small protein shake or yogurt with fruit) probably *won't* make you gain weight. On the other hand, this new study controlled for all kinds of factors that could potentially affect the results, like how healthy the food was and how much exercise the subjects were doing. That means that these results hold for people with healthy habits, too, not just those who are eating indulgent foods before bed.

It's unnecessary to change your habits if you're happy with your weight and your overall health. But if you're concerned about weight gain, cholesterol, or any of the other factors that were negatively impacted during this study, it may be worth it to try adjusting your eating schedule to focus more on daytime to see if it makes a difference for you.

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