I would personally claim to be a short sleeper. Without any testing to back it up, however. I regularly sleep from 2am to 6am, and have pretty consistently for 25 years or so. It was worse in high school. 12am-2:30am, but that was heavily influenced by gaming and came with some evident drawbacks.<p>If I were to make any guesses surrounding my personal experience, I would assume that I fall into REM remarkably fast. When I lay down I'm usually out and in a deep sleep within 15 minutes. To the ire of my family, as I'm a heavy snorer. When I wake I'm very quick to being fully alert and I typically don't nap.<p>Just wanted to add my tidbit of anecdata.