That may depend on the type of apnea (central, obstructive or complex). Position does have an effect, but they want you to sleep as close to your normal pattern as possible, so if that includes sleeping on your side, so be it.

My wife & I both had two go-arounds with the sleep lab - once preliminary, and once with a trial c-pap. Each time they told both of us to sleep however we normally do, which makes sense. I couldn't the second time, but the way they conducted the whole test was so unscientific and with so many variables that it was a joke. They didn't seem to care. I was tempted to write about it, but to what end/purpose?

They still dxed us both with apneas sufficient to warrant c-paps.

Doc