Sleeping with a pillow, without a pillow, with a thick soft pillow, with a thin firm pillow and so on have been topics of endless debate. There are multiple factors which determine which you choose for yourself including medical opinions, personal preferences, health considerations and even cultural propensity.
How is posture affected by a pillow?
While the pillow debate continues the one point which has been argued medically is that a thick pillow for someone who sleeps on their back causes a misalignment of the cervical vertebrate by pushing the head up all night. This over-curvature of the vertebrate becomes permanent over time and that affects posture. A thin pillow, however, does not cause the damage since there is a small natural curvature to the spine which is accommodated by a pillow that is less than 6 inches thick. Also this means that if someone sleeps on their side a pillow does not impact their spines and does not affect their posture.
Is there any harm to using pillow?
Some people claim that pillows create an unnatural incline for the body and limit how the person is positioned during sleep. Sleeping without a small amount of support under the neck is, however, uncomfortable and can cause an increase in snoring through the night. If someone uses multiple pillows they may train their body to slouch around the pillows which will cause back problems and result in bad posture. A thin pillow seems to pose no negative effects though – and a pillow that aligns the neck with the chest and lower back is ideal.
Conclusion
Sleeping without a pillow, provided you sleep flat on your back prevents you from slouching and therefore keeps your posture straight but using a thin pillow causes no harm even if you sleep on your back. If you do not sleep on your back, using a pillow has no impact on your posture.