I have sleep apnea that I treat with a BiPAP machine. The neurologist who manages it said that everybody has sleep apnea to some degree, and, as a matter of fact, with my BiPAP machine, I have fewer episodes than the average person.
Sleeping on the left side allows the stomach to operate better while sleeping on the right favors the heart. I'd always heard this, but recently read a short article confirming it. Dunno if it's true, though.