I found it interesting that Steve Pavlina actually did a planned experiment, he did each duration 8-10 times“25 Minutes. So far I’ve tested countdown alarm times of 20, 22, 25, and 30 minutes for my naps, trying each one for about 8-10 cycles. I begin the countdown timer as soon as I lie down. 25 minutes seems to work the best so far. With shorter times the alarm sometimes cuts my sleep too short, and I don’t feel fully refreshed. With longer times I often feel groggy upon awakening if I sleep until the alarm goes off, probably because I’ve slipped beyond the REM phase. 25 minutes seems to be the sweet spot that gives me enough time to fall asleep, have a dream, and wake up naturally if possible, with the alarm serving as a safety net to prevent me sleeping past REM. I notice that even if the 25-minute alarm wakes me up (which is about 1/3 of the time now), I usually wake up feeling relaxed and refreshed and remembering a dream.”
http://www.stevepavlina.com/blog/2005/11/polyphasic-sleep-log-days-12-18/