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Mice are nocturnal creatures. They eat during the night and sleep during the day. If, however, you just feed mice during the day, they gain more weight than if you feed them a similar amount of calories at night. Same food, about the same amount of food, but different weight outcomes, suggesting that eating at the “wrong” time may lead to disproportionate weight gain. In humans, that would presumably mean eating at night.

Weight management recommendations often include advice to limit nighttime food consumption, but this was largely anecdotal until it was first studied experimentally in 2013. Researchers instructed a group of young men to not eat after 7pm for two weeks. Compared to a control period where they continued their regular habits, after the night-eating restriction they ended up about two pounds lighter. This is not surprising, given that dietary records show they inadvertently ate fewer calories during that time. To see if timing has metabolic effects beyond just foreclosing eating opportunities, you’d have to force people to eat the same amount of the same food, just at different times of the day. The U.S. army stepped forward to carry out just such an investigation.

In the first set of experiments, army researchers had people eat a single meal a day either as breakfast or dinner. The results clearly showed the breakfast group lost more weight. Check it out: Have people eat only once a day at dinner, and their weight doesn’t change much. Have them eat once a day at breakfast, and they lose about two pounds a week. Like in the night-eating restriction study, this is to be expected, given that people tend to be hungrier in the evening. Think about it. If you went nine hours without eating during the day you’d be famished, but people go nine hours overnight all the time and don’t wake up ravenous. There is a natural circadian rhythm to hunger that peaks at about 8pm and drops to its lowest level at around 8am. That may be why breakfast typically is the smallest meal of the day.

The circadian rhythm of our appetite isn’t just behavioral, it’s biological. It’s not just that we’re hungrier in the evening because we’ve been running around all day. If you stayed up all night and slept throughout the day, you’d still be hungriest when you woke up that evening. To untangle the factors, scientists use what’s called a “forced desynchrony” protocol where they lock people up in a room without windows in constant unchanging dim light and make people sleep in staggered 20-hour cycles to totally scramble them up. This goes on for over a week, so the study subjects end up eating and sleeping at different times throughout all phases of the day. Then you can see if cyclical phenomenon are truly based on internal clocks or just a consequence of what you happen to be doing at the time.

For instance, there’s a daily swing in our core body temperature—and blood pressure, and hormone production, and digestion, and immune activity, and almost everything else. But let’s use temperature as an example. Your body temperature bottoms out usually around 4am, dropping from 98.6 °F down to more like 97.6 °F. Is this just because your body cools down as you’re sleeping? No, you can show experimentally, by keeping people awake and busy for 24 hours straight, that it happens at about the same time no matter what. It’s part of our circadian rhythm, just like our appetite. It makes sense, then, if you are only eating one meal per day and you want to lose weight, you’d want to eat in the morning when your hunger hormones are at their lowest level.

Okay, but then things start to get weird.

The army scientists repeated the experiment, but this time they had the participants eat exactly 2,000 calories—either for breakfast or for dinner. That takes appetite out of the picture. They were also not allowed to exercise. Same number of calories, so same change in weight, right? No, the breakfast-only group still lost about two pounds a week compared to the dinner-only group. Two pounds of weight loss eating the same number of calories. That’s why this concept of chronobiology, meal timing—when to eat—is so important.

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