I actually have exactly this same issue, same position too. I actually can fall asleep in other positions and sometimes feel like resting in other positions, but if i'm sleeping at night i cant get into deep sleep unless i lay that way. Weird. I feel like I am in a slightly different states of mind in different positions. For me there are basically three, stomach facing left, stomach facing right, and laying on back. I cant sleep on my back too long because of sleep apnea.
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That is not dead which can eternal lie,
And with strange aeons even death may die.