No. I know many projectors (from forums) who write that they also are lying on the side or that it does not matter. This includes myself as I am a side-sleeper and cannot lie on my back at all for falling asleep or deep relaxation.
Sitting in a chair (as recommended by RB) would in my view only be sth for very experienced projectors who can induce trance easily and from waking state. It is fine for meditation though, also for me. Just my 2 cents.
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