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You think of your sleep position as a preference. It is closer to a setting, and it changes what your airway and your stomach do all night.
Start with what to stop, which is sleeping flat on your back. Face up, the tongue and soft tissue of the throat fall backward, and the airway narrows. Snoring and breathing pauses both climb when you lie on your back. There is less holding the airway open against gravity.
Studies of breathing by position find events roughly twice as common on the back as on the side. For many people, getting off the back is the single largest change available.
So the first answer is side over back, for anyone. The second answer, left over right, applies specifically if your problem is reflux. Lie on your left and the stomach sits below the junction where it meets the esophagus. Acid has to climb against gravity to come up, and less of it does.
Lie on your right and that junction sits low, near the pool of stomach contents. Acid reaches it more easily, and the burning and the 3am waking follow.
The study here looked at sleeping position and nighttime reflux directly. It found the right side associated with more acid exposure and reflux at night than the left.
It was an observational study, so it shows the association rather than proving cause. The mechanism, gravity and the angle of the stomach, is well understood and points the same way.
TONIGHT
Here is the move. Get off your back first, whatever else you do. Put a pillow behind your back so you cannot roll flat, or sleep pressed against it. Keep the neck neutral, not cranked up on a stack of pillows.
If reflux is the issue, the burning, the sour taste, the 3am wake, choose the left side. If it is snoring or breathing, either side beats the back and which side matters less.
Give it a few nights. Rolling onto your back in your sleep is the main obstacle, and the pillow behind you is what handles it.
Position is the rare lever you set once and it holds all night. Off your back, and onto your left if it burns.
