As you sleep, the pituitary gland releases growth hormone, which in turn stimulates your body to produce insulin-like growth factor—hormones essential for muscle repair, reports Shape.

If you’re working out intensely— training for a marathon, for instance—you may need an hour more for proper muscle repair and recovery, says John Underwood, the director of the Human Performance Project.

In fact, less than eight hours can actually shrink your muscles, research shows.