The best power nap occurs for whatever length of time it takes before you just hit REM sleep. For most people, that's between 90-120 minutes. Anything less than that and it's less effective. For me, I need at least 90 minutes before I awake refreshed and alert. One hour doesn't work.
Stages of Sleep
Stage 1 - Drowsiness, lasts from 5-10 minutes.
Stage 2 - Light Sleep, lasts from 20-30 minutes, less if very tired, more if less tired.
Stage 3 - Deep Sleep Transition, lasts from 5-15 minutes, low end if very tired.
Stage 4 - Deep Sleep (Delta Sleep), lasts for 30-40 minutes.
It's at or near the end of Stage 4 is where the effectiveness of the power nap is to be found. After that is REM sleep. However, Stages 3 and 2 repeat backwards before REM sleep is attained. The normal sleep cycle is 1, 2, 3, 4, 3, 2, REM. For most people, that's 90-120 minutes before REM sleep is reached. While you may think a one-hour power nap works, a large part of why it works is because one hour is a nice even round number, so people think it works. The scientific reality is that it takes (most people) at least 90 minutes, but no more than 120 minutes, to get the most benefit from that power nap.
On the other end of the scope of things is how long can you sleep before you need to finish it out? For most people, that's 4 hours. Anymore than four hours and your body will demand it's allotted 8-9 hours and you'll spend the rest of the day groggy and juuuuust a little out of it.
People who vehemently declare that they only need 4 hours of sleep a night are kidding themselves (they certainly crack me up). They are the ones who get tired mid-afternoon, get drowsy while driving, sometimes after very short distances. These are the people who proudly run the week on 4 hours a day, then crash on the weekends, sleeping most of it away. Or they get sick a lot, with mild flu-like symptoms, and then wonder where they caught the bug from.
If they defend the 4 hours a night by saying that they can only sleep that long, that every time they go to sleep they wake up 4 hours later, they're still going through life tired, whether they will admit it or not. These people need to go to a sleep clinic and git er dun.
Slow and steady, even in expediting, wins the race - Aesop