Do you wake up with heartburn, a cricked neck, and a backache? Or maybe it’s more subtle: you wake up feeling tired, like you haven’t really slept. Surprising new research suggests your sleeping position may be to blame. Most people have a preferred position, a default that feels the most natural. But just because something feels natural doesn’t make it healthy. Here’s a run-down of the best and worst sleeping positions, and what you can do to make them more comfortable.