I always start on my side; I don’t have a preferred one. I always seem to be on my back when I wake up.
Thanks to a lot of sinus congestion, when I am awake I often roll from one side to the other to try to drain the side on top. I have fantasized about a bed that rotates like a rotisserie so the sinuses don’t have a chance to get clogged up from gravity.
If you build that bed we will buy one. Both DH and I have the same sinus issues and the rotating bed would be GREAT.
Left side, unless I forgot to do my shoulder exercises and my shoulder hurts. Then right side. Sometimes on my back. Never on my stomach. My parents have pictures of me as a toddler sleeping on my back, sort of a starfish position. Now I keep my limbs a bit closer, as my husband is more of a bed hog than me.
Flat on my back, no pillow, with my hands folded on my stomach corpse-style. 
I do sometimes fall asleep while reading, and that means on my right side, with a pillow, because the light is there. But I’ll wake briefly when my SO comes to bed, removes my book, and turns out the light. Then I ditch the pillow and turn onto my back.
Kickstarter, here we come!
I start on my back, then switch over to the left. Can’t sleep on the right, as it gives me acid reflux.
Any position is good when there are no cats on the bed.
Horizontal. I cannot sleep sitting up or even on one of those chairs they give you in the hospital if your with a patient.
I sleep in the recovery position but tending a bit more to my stomach than my side.
I was just going to come in here and say that I sleep like one of those hotdogs in the 7-11, constantly rotating.
On either side, semi-fetal. I like a small pillow between my knees to avoid bone-to-bone contact, and wad up the covers between my ankles to prevent bone-to-bone conatct. I also like another pillow to to semi-spoon with; to give my upper arm ( the one that’s not against the bed ) support/sling. Wad up bedding slightly so that my lower ( the arm against the bed ) wrist is slightly elevated. To top it off a pillow over my head with the open end flap tucked between my head and the pillow, with just my nose/mouth exposed.
If I was given a bed with just one pillow and minimal ( thin ) bedding: I’m in trouble.
When I lay down for the night, I’m not unlike a corpse in a coffin – on my back, hands over my abdomen. I’ll toss & turn throughout the night but I generally stay on my back.
Least is on my stomach; I never start or wake up there. Favorite to fall asleep if left but usually I wake on my right or back.
Why isn’t “On a big pile of money, surrounded by many beautiful ladies” listed as an option?
I had a pic of me sleeping in the kneeling position you see little kids doing sometimes.
I also sleep in recovery position - both right and left, precessing through on the stomach.
Count me as another “No, I don’t”.
On a large pile of money surrounded by lots of beautiful ladies.[/Wolfcastle]
Supine. Unlike some cattle, I can’t sleep well standing up.
Left side, arm around Madame Pepperwinkle.
I have to start off on my right side, but will wake up on either side or on my stomach. Never on my back.
Occasionally I roll onto my stomach and pin both my arms and they go numb. Trying to roll back over without use of your arms is fun.
And once last year I had a vivid dream about having repeated asthma attacks and gasping for air (I don’t have asthma). I woke up to find I had been sleeping on my stomach with my face directly into the pillow in such a way that it was blocking my nose and mouth and I was sort of suffocating myself. I guess my brain sounded the alarm and woke me up. That was a bit unnerving.