If you’re used to eating late dinners or snacking well into the night, you might be keeping your body from burning fat while you sleep, according to a study published in the journal PLOS Biology by a research group in Vanderbilt University.

Their findings imply that, when it comes to getting your weight under control, when you eat matters as much as what you eat and how much exercise you get.  “If you take a late night snack, when you go to bed you will be preferentially burning carbs, and that delays the time in which you flip into [fat] burning mode,” says Carl Johnson, professor of molecular physiology and biophysics at Vanderbilt University, and senior author of the study.

The goal was to determine whether food is metabolized differently depending on when you eat the bulk of your calories. Human participants aged between 50 and 65 were divided into 2 groups: The first group ate a 700 kcal breakfast in the early morning, whereas the second group skipped breakfast and instead had the same 700 kcal meal as a late snack at 10 pm. 

Study participants ate a 700 kcal meal consisting of English muffin/margarine, fresh pork patty, honey nut Cheerios, mandarine orange, cranberry juice/ 2% milk, a decaf coffee or decaf tea. Participants were divided into 2 groups: The first group ate this meal at breakfast at 8 am, while those in the second group skipped breakfast and had this same meal at 10 pm

The participants in both groups consumed the same food and the same amount of calories per day— they both had lunch at noon and dinner at 6 pm— and had an overnight fast of approximately 14 hours. They stayed in a metabolic chamber for 56 hours where they ate and slept. From this chamber, the researches could measure the rate at which they were burning fat vs carbohydrates.

“[The metabolic chamber] is a really cool method where we can continuously monitor metabolism,” says Kevin Parsons Kelly, a postdoctoral fellow in the Johnson lab and first author of the study. He explained that the chamber is sealed with a set flow rate of oxygen and CO2 where they were able to monitor how much oxygen each participant was taking in and how much CO2 they were taking out. This is known as the respiration rate.

“We can actually learn a lot [about metabolism] just from how a person is breathing,” says Kelly.  When you break down [carbs and fat] through normal metabolic processes, they require different amounts of oxygen and CO2. Once we know the respiration rate, we can know how much carbs versus fats are being burned,” he explains.

They found that those who ate the 700 kcal meal at breakfast and finished their last meal at around 6 pm switched to fat burning mode while they were asleep, while those who ate the late night snack primarily burned cabs just before going to bed and throughout the night.

Eating the bulk of your calories during the day helps you burn fat as you sleep

Johnson explains that we have an internal clock that regulates when our bodies switch from primarily using carbs to primarily using fat to create energy, but this cycle goes out of whack when we eat late at night. “If you take that snack just before you go to bed, the carbohydrates are easier to burn and it delays going into [fat-burning] mode,” he says.

During the 56 hours of the experiment, the researchers measured the levels of physical activity of the participants and the amount they slept and found that both groups slept about the same amount of hours and had similar levels of physical activity through the whole experiment, indicating that the differences in fat burning rates were the result of when they ate their meals.

The switch to fat-burning mode occurred mostly as the participants were sleeping, and there was no difference in fat burning rates between the two groups during the morning. “Breakfast skipping had a much more minor effect than having already been fasting for a few hours [before bed],” says Johnson. 

The study was too short to determine whether eating earlier in the day leads to any weight loss, but Kelly explains that in a more longterm study they would expect to see higher fat reduction and likely reduced weight in those who consume the majority of their calories during the day.

“It’s important to stop eating after supper,” says Johnson.“ It would be good if there’s been three or four hours [after your] last meal before you go to bed so that you can really flip into that fat burning mode as you sleep.”