The spine is a column of 24 individual bones called vertebrae that support the body's weight and protect the spinal cord. Spinal disks, located between the vertebrae, hold the spine in shape and act as shock absorbers. The spine has three natural curves if viewed from the side of the body. These are the cervical (neck area) curve, the thoracic (upper back) curve and the lumbar (lower back) curve. These three curves give the spine an "S" shape when viewed from the side of the body. When viewed from the front or back of the body, a healthy spine forms a straight line. Good posture refers to a body position that keeps the spine in the natural shape described above. Good posture reduces strain on the muscles and ligaments of the spine, and prevents muscular pain(9). Unfortunately, common sleeping positions often lead to poor sleeping posture and neck and back pain when sleeping(1,4,6,7).
The side sleeping position is considered the most popular. In this sleep position, the spine maintains its natural S shape, which promotes good sleep posture. Normal side sleeping does have some disadvantages though. When the body is lying on its side, gravity pulls the spine from its naturally straight shape (when viewed from the front or back of the body). This can lead the neck and lower back regions of the spine to bend toward the sleep surface and cause neck or back pain when sleeping. Side sleeping may also result in lower back strain due to over-rotation of the pelvis.
This sleep position is the second most popular as it is preferred by approximately 30% of people. The typical back sleeping position places strain on the lower back and neck. Normally, the buttocks and the upper back are in contact with the sleep surface, while the lower back is minimally supported. This posture can strain the lower back and cause pain and discomfort. Traditional pillows often provide minimal neck support and induce neck strain in this position.
For adults, this sleep position places the greatest degree of stress on the spine. Perhaps for that reason, it is the least common. Stomach sleeping, especially on soft surfaces, forces the lumbar curve of the lower back into a flattened position. This is known to cause muscle strain and can lead to lower back pain. Stomach sleeping also forces rotation of the head which can result in strain to the neck. Medical professionals often encourage stomach sleepers to try to adopt another sleep position for these reasons.