Sleeping is supposed to be rejuvenating—so why do you wake up so stiff all the time? Simple: it’s because you’ve just spent eight hours in one position, says Andrew Gitkind, MD, medical director of the Montefiore Spine Center. That’s why it’s so important to sleep in a position that won’t aggravate any existing aches, especially if you struggle with lower back pain, says Jennifer L. Solomon, MD, board certified physiatrist in physical medicine and rehabilitation at the Hospital for Special Surgery. (And if you haven’t already dealt with a sore back, there’s a good chance it’ll hit you at some point: 80% of adults will deal with back pain in their lifetimes, according to the National Institutes of Health.) Here’s exactly how you should sleep—and how you shouldn’t—to wake up refreshed and back pain-free.