Humans have not just one body, but multiple ethereal bodies that occupy the same space as our physical form; here, we'll discuss three known dimensions relative to the human being.
The human body is a magnificent biological-machine, so we need not go into detail over it's anatomical wonders; however, we'll briefly shed light upon it's spiritual centers.
Based upon my observation and experience, the longer you remain in RTS during a body-split, the more deplete of ethereal energy you become; this is why I only use this method for "serious" purposes.
Ever had a dream that felt so real, you didn't know if you were awake or sleeping? If so, then you've probably visited one of these places before.
Remember that erotic dream that caused you to..well--never mind that. Here's a list of 5 Astral Beings encountered during dreams.