Carmen, I phsyically can't sleep on my left side throughout the night, try though I do! My hip ends up killing so I invariably shift over to my right side at some point. After a while my right hip starts hurting from the pressure so I flip back onto my left side! I figure, as long I am getting sleep and recharging my batteries, that this is more helpful to bub and me than trying to stay in the one position all night long and literally losing sleep over it - it's hard enough to sleep in one position normally, let alone when you're not pregnant!
I was talking to my mum about this left side sleeping thing the other day and she said "oh I slept in any old position with you and your brother" - and we turned out just fine!