There are a million diets out there — some more effective than others. But at the end of the day, it all boils down to one thing: if you want to lose weight, calories in must be lower than calories out. It’s as simple as that.

Or… is it?

Simple math?

The calories in vs calories out calculation is essentially correct, but it leaves out a few things. For starters, having a healthy diet isn’t only about calories — ignoring nutrient intake is not a wise thing to do, and many diets sacrifice healthiness for the same of losing weight (I’m looking at you, keto).

Secondly, there’s the human aspect: we’re not exactly good at controlling ourselves, and the odds are that simply making a calorie plan isn’t going to work out. It needs to consider our lifestyle and preferences in order to be successful. A diet needs to become a lifestyle in order for it to work.

Lastly, it’s also not only about what we eat and how much we eat, but also about when we eat, as a new study has shown.

Our bodies are not machines that you can program and expect to function in the same way day in and day out. Our biological clocks and sleep patterns (the circadian rhythm) regulate how the food we eat is metabolized. Our bodies do burn fat when we sleep, but our metabolism generally tends to slow down at night, so eating right before going to sleep is rarely a good idea.

In the new study, researchers monitored the metabolism of mid-aged and older subjects in a whole-room respiratory chamber (essentially a calorie-measuring room) over two separate 56-hour sessions.

In each session, lunch and dinner were offered at the same time: 12:30 and 17:45, respectively — but the timing of the third meal was changed. For one half of the study, the additional meal was presented as breakfast (8:00), and the other as a late-evening snack (22:00).

The duration of the overnight fast was the same overall, it was just the time of eating that was changed.

The two sessions did not differ in overall energy expenditure, but the respiratory exchange ratio was different during sleep.

Essentially, while the subjects were consuming the same amount of energy during sleep, they were metabolizing nutrients differently. Specifically, they were less likely to burn fat during sleep when they had the late-night snack

“Therefore, the timing of meals during the day/night cycle affects how ingested food is oxidized or stored in humans, with important implications for optimal eating habits,” the researchers write.

Previous research has suggested that this downside is negated by an intensive training regime, but for most of the population, the takeaway is pretty clear: if you want to get rid of that extra fat, you might want to stop having late-night snacks.

The results were published in PLoS Biology.