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.

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.