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Probably more health zeitgeist of the '90s than any real concern. Your bed, body type, and sleep style might make one more appealing than the other.

I was a restless sleeper though and had/have extreme difficulty getting to sleep often; learning to sleep on my back helps. Expressed another way, if my brain says I have to be on my side for comfort, and I have to achieve comfort to sleep, then if that comfort isn't achieved no sleep for me. Getting my brain to accept "and now we lie down for sleep" without thinking about comfort let me be actually comfortable in more positions/settings.