If you cut your sleep down to just two-thirds of what you normally get for eight days straight, you’ll likely eat about 559 extra calories a day without moving more—this could be why people who don’t sleep enough tend to gain weight.

When people don’t get enough sleep for eight days, they don’t end up moving more to burn off the extra calories they eat—so they’re likely to gain weight just from eating more, not from being lazy.

If you sleep less than 7.7 hours a night, the less you sleep, the higher your body mass index (BMI) tends to be — for example, people who sleep 5 hours have a 3.6% higher BMI than those who sleep 8 hours, even when you account for their age, gender, or breathing issues during sleep.

When young men slept less, they burned a little more energy during the night because they were awake longer—but this extra burn didn’t add up to more total energy burned over the whole day.

If you don’t get enough sleep for 8 days, your hunger hormones don’t change much—so if you end up eating more, it’s probably not because these hormones are telling you to.

Even after sleeping only 3.5 hours a night for three nights, young men burned the same total amount of energy and used the same mix of carbs and fats as when they slept 7 hours.