I came across this blog post discussing the importance of understanding sleep cycles for the purposes of recovery and productivity. It makes an interesting case for understanding how your body responds to waking during the various stages of sleep over an 8 hour period. It indicates that at the start of a sleep there are two periods of approximately two hours long each where you typically progress through 4 stages (depths) of sleep and back up to REM sleep. Each period lasts about 2 hours. Following this, your cycles into periods of sleep get progressively shallower in depth (and shorter) until you awake uninterrupted. You may even awake briefly during the REM stages of sleep, only to fall back asleep again. By the time you reach around 5 to 6 hours of sleep, you are only cycling through 2 stages of sleep during each period. The article makes the argument that it is best to wake from a shallower level of sleep than a deeper level, and sometimes that may only vary by 10-20 minutes.
Look at the sleep cycle chart. I can see a few key times that would be worth testing on yourself. First and foremost, it seems like if you are falling asleep, then within about 20-30 minutes you’ll drop into a fairly deep level of sleep. If you are trying for a quick nap to rejuvenate, then keep it short and under 20 minutes. Want to know more about naps and the physiological and neurological benefits of napping? Refer back to this January 2014 post by Andy and try it on for yourself. Remember that you are your own case study in life. Read, test, and see how it works on you!