@StarryGwee summed it pretty well. I'll add my two cents here.

First off, SP usually happens upon waking up. Entering SP intentionally from a waking state is not always easy to do. Let's face it, in a sleep cycle you usually starts with NREM-1 and NREM-2. These are stages were your body is not yet fully shut down!

IMO, a lot of the time when we LDers say SP, we are not really talking about the authentic SP. What we experience is mind/body separation, which feels like SP but in a strict sense it is not.