Don Herbison-Evans ,
(updated 29 June 2014)
Sleep apnea is a condition where the airways between nose and lungs of a person are inclined to close during sleep. This can cause sleep disruption and even oxygen starvation, as well as noisy breathing, disturbing one's partner.
The key to controlling apnea in some people is to imagine that there is a section of the windpipe that has a 'D' shaped cross-section. The straight side of the 'D' is against, and held in shape by the spine. The curved part of the 'D' is held open unconsciously by muscles while one is awake. But when one is asleep, these muscles relax.
If one is lying on one's back, these relaxed muscles are inclined to flop against the back of the windpipe, thus restricting the airflow. The windpipe can be kept open by avoiding lying on one's back, and lying instead on one side. It does not matter which side for the moment, so consider lying on one's left side.
When lying on one's left side, it is important to keep the head in line with the spine, so the head must be supported by one or two pillows, so that the head is basically lying on the left ear. The shoulders must now be rolled forward so that the chest is facing slightly downwards in order to open the relaxed muscles of the windpipe. About 10 degrees roll to the left is sufficient for these muscles to flop open under gravity, and thus keep the windpipe open. The head can be held twisted slightly to the right to keep the nose out of the pillow.
Having twisted the body and head this way, there is a need to maintain this position when asleep. This can be done by shifting the top leg (in this case the right leg) forward, and shifting the lower (left) leg back. This twists the pelvis so that this is also facing slightly downwards, by the same 10 degrees, so that the torso actually has no twist. The weight of the right leg being ahead of the left keeps the body facing slightly downwards while asleep, and so keeps the airways open.
Some people find that when lying on one side, one nostril of the nose is inclined to block with mucus from the sinuses. In this case, some people find paradoxically that breathing is easier when lying on the side of the unblocked nostril. So if the right nostril is blocked, lying on the left side, as described above, makes breathing easier. But if the left nostril is blocked, they find that it is easier to breath while lying on the right side, with the left leg forward and the right leg back (the opposite of the posture described above) to maintain the airways open the while asleep.