The sleep thing might also be explain by sleep cycles. Sleep cycles are roughly 90 minutes long. If you wake up at either the beginning or end of a sleep cycle, you feel more refreshed than waking up in the middle of one. So it's usually multiples of 90 minutes. If your brain/body feels like it needs more sleep, than it will go on to the next sleep cycle. Until an alarm goes off and you're forced to wake up no matter what, or until you feel rested enough and you'll wake up naturally. Naps should either be combinations of 90 minutes, or it can be 20 minutes exactly. 20 minutes is like the first part of the sleep cycle, I forgot what occurs in that first part that's different from everything else, but it's a point where you can wake up and feel well rested. Possibly why Mr. Sheele recommends 20 minutes or a sleep cycle(90 minutes) for the information to incubate.
