The halacha is that it is forbidden to sleep on one’s back or stomach. Is one permitted to simply lie down on his back or stomach without going to sleep?
Is one permitted to do exercises that require lying on the back, such as sit-ups or flutter kicks? If one knows that lying on his back and doing these exercises might cause him to become aroused (a man), is he forbidden to do this?
Although there is a dispute as regarding lying on the back or stomach, never the less one can rely on the more lenient opinions. As far as doing exercises, if they might cause him to become aroused he should refrain from doing so.
Brochos 13b, E:H 23:3, O:CH 239 Mishnah Berurah 6, Shoiney Halochos 239:3 & Orchos Rabenu 3:163, Oz Nidberu 6:50, Chidushim Vebiurim Berochos 13b, Birur Halocho 239.