The theory is that after a couple of days of you short circuit the natural sleep cycle, to go into REM sleep instantly when you sleep. As we only experience about 2 hrs of REM sleep per night, you get all of the sleep you need in these 15-20 min bursts, every 4 hrs or so. Think how when you’re really tired, a little nap will fully refresh you for a few hrs., but sleep a little longer, and your fucked!