It depends on your body's sleep cycle. However long it lasts in the first couple stages of sleep is what you want. Last thing you want to do is wake up in the middle of a deeper level of sleep. Though since it does cycle, in theory its just as good to wake up right -after- a deep/rem sleep. Which is probably why some of these people are saying they feel great after an hour. And others are saying they feel terrible after an hour. People cycle at different rates. If you can figure out your rate, you wanna wake up in a lighter level of sleep. You can use this method for your nightly sleep too. I think my cycles are about an hour long too 'cause I can get four hours of real sleep in a night and feel fine. But if I get six and a half I feel like crap. And 10-15 minute power naps are pretty good for me. But I like to take a real (deeper) nap when I can