 There are appropriate postures for standing, appropriate postures for sitting, and appropriate posture for lying down. They're each a bit different. Yeah, so your head on average weighs about 11 pounds, and right now you're holding it out in front of your body. So shoulders should be not rounded forward, but instead back and down. Chin should be relatively tucked, keeping your ear just in line with your shoulder. Notice also that my shoulders are stacked just over my pelvis, over my hips here. It's not too far forward or too far back. My knees ideally would be at a 90 degree angle, displacing my weight equally over each of my hips. My feet would be flat on the ground. Everything else stays the same. My shoulders should be back and down, ears over my shoulders. What she's doing by lying face down is putting her spine into slight extension, and that's actually compressing some of the joints that you'd find along the posterior aspect or the back of your spine. Putting the knees into a slightly flexed position are going to make the lower portion of your spine a bit more neutral. The idea is that we're supporting the curves that should be there and not allowing any abnormal curvatures or exaggerations of curves to persist throughout the night. It's important to note that posture is going to change slightly depending on the person's build. We're all a little bit different. You can always correct your posture. It's never too late, and it's always going to help if you do correct it. You may not be able to reverse the damage you've done in certain cases if it's gone on for years and years and years. However, you can prevent future damage, more damage occurring in the future.