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, 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.

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.

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, 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.

The heart's ability to pump blood per minute per body area remains stable even after 32 hours without sleep in healthy men.