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.

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.

When young men slept less, their bodies released more of the stress-related chemical norepinephrine, especially at night and in the morning, which may be a sign their stress system was overactive.

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 young men slept less, their body’s internal temperature dropped slightly over two days, suggesting their body’s natural daily temperature cycle was disturbed.

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.