This happens to me a lot! I usually get in full SP but I don't succeed in falling asleep. I think it's really frustrating, but KingYoshi's tutorial helped me a lot, especially with the reversed blinking, which made me sleepy. I still fail at sleeping on command, but I managed once to do this, but no WILD, though.
Oh, and I think you SP more when you lie on a side because you just feel more relaxed.