A subset of my patients with obstructive sleep apnea (OSA) have it only on their back. By this, I mean that I've documented with a sleep study that a patient has OSA when sleeping on their back, but no OSA when on their sides. In my experience, this type of OSA is rare and usually only in patients with normal weight. But these patients have a treatment option that does not work in a majority of OSA patients - positional therapy. This means that if the patient can sleep on their sides or stomach only, they greatly reduce their snoring and sleep apnea. The trick is to not sleep on their back - something hard to do if they are asleep! I usually advocate using pillows behind the back to keep the person from rolling onto their side.
The FDA has just cleared an interesting device for positional OSA. It's called Night Shift by Advanced Brain Monitoring. It's a small device worn around the neck at night. It detects position (probably by gyroscope) and causes mild vibrations when the wearer is on their back. The vibrations help the user roll over, but stay asleep. The device vibrates more vigorously if the person is not responding. Their have been some studies on this device and they report improvement in sleep quality, reduction of snoring volume, and reduction of OSA.