Rav Binyomin Zilber - Waking Up Someone Sleeping In The Wrong Position
A man may not sleep on his back or his stomach, but must sleep on his side. Someone asked Rav Binyomin Zilber (Az Nidbiru 12:42) two questions. First, if you see someone sleeping in a halachicly improper position should you wake him up to get him to lie himself correctly? Second, does this apply only when sleeping or even lying down while you are awake.
Rav Zilber answered that the issur is to go to sleep lying flat. Once a person is sleeping he is not oveir any issur and is not responsible to be in any specific position. Therefore you need not wake him up. However if he wakes up and finds himself in that position, he must shift his body to its side.
With regards to lying flat while awake Rav Zilber says that it is permissible and this din only applies to sleeping.
Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha. We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.