The question of whether to sleep with one or two pillows is not about comfort alone, but about positioning the head and neck to support the spine. The primary function of any pillow is to maintain the natural alignment of the cervical spine, the seven vertebrae in the neck. Failing to choose the correct pillow thickness, also known as loft, for your specific sleeping posture can introduce strain and potentially lead to morning stiffness or pain. This article clarifies the scientific goal of pillow usage and details how to select the right number of pillows for optimal sleep health.
The Goal: Maintaining Neutral Spinal Alignment
The body requires the head, neck, and spine to form a continuous, straight line while resting to minimize stress on the soft tissues and joints. This posture is referred to as neutral spinal alignment. The pillow’s role is to fill the gap between your head, neck, and the mattress, preventing the neck from angling unnaturally upward (hyperextension) or downward (flexion).
If the pillow is too thin, the head tilts down; if it is too thick, the head is pushed up. Both scenarios disrupt the neutral position of the cervical spine. Maintaining alignment allows the muscles and ligaments in the neck to fully relax and recover. The correct loft depends entirely on the distance between your head and the mattress, which changes with sleeping position.
When One Pillow is Recommended
For most people who sleep on their back or stomach, a single, specialized pillow is the appropriate choice to achieve neutral alignment. Back sleepers generally require a medium-loft pillow that supports the natural inward curve of the neck without pushing the head too far forward. The ideal thickness is often between 4 to 6 centimeters, allowing the head to remain on an even plane with the rest of the spine. The pillow should cradle the neck and head, but the shoulders should rest on the mattress surface, not on the pillow itself, to avoid misalignment.
Stomach sleeping is discouraged by specialists because it forces the head and neck into prolonged, severe rotation, straining the cervical spine. If this position is unavoidable, the pillow must be extremely thin, ideally less than 5 centimeters thick, to minimize the angle of rotation. Some stomach sleepers find that using no pillow, or a very flat one, is the only way to reduce strain. A flat pillow may also be placed under the pelvis to help maintain a neutral curve in the lower back.
When Two Pillows Are Recommended
The need for a second pillow is most often associated with side sleeping or for supporting the lower body, not for stacking under the head.
Head Support for Side Sleepers
Side sleepers have the largest gap to fill—the distance from the ear to the outside of the shoulder—which demands a higher loft than other positions. This height is achieved by using one thick, firm pillow, or by stacking two thinner or medium pillows to create a combined loft of approximately 10 to 15 centimeters. The goal is to ensure the neck is level with the rest of the spine, preventing the head from drooping toward the mattress.
Body Alignment Support
The second common use for a second pillow is for body alignment, separate from head support. Side sleepers benefit from placing a firm pillow between the knees, which prevents the upper leg from rotating and pulling the pelvis and lower spine out of alignment. Back sleepers can place a pillow beneath their knees to slightly flex the joint, reducing stress on the lower back and improving overall spinal alignment. In both cases, the second pillow supports the torso and pelvis, allowing the primary head pillow to focus on the cervical spine.