Read this carefully: I used to think this way as well. This thinking lead me to the exact wrong strategy for naps.

There are many stages of sleep; stage 1 through 4, and REM sleep. These, it is hypothesized, have distinct physiological functions. Most importantly for this discussion, waking up out of each have very different effects.

Waking from stage 1 sleep makes you feel immediately refreshed, and people perform better immediately have tests of mental acuity. This is also true of REM sleep; additionally you often remember dreams.

Stage 2 is also good. A good 30 minutes afterwards, you will feel better rested, and your mental acuity will rise; for longer than from stage 1, but it is a delayed effect.

But if you wake up out of a deeper sleep cycle, you get 'sleep inertia.' Mental acuity plummets. You feel poorly rested and ill.

The problem:

The body is often not even aware that you are sleeping in stage 1 sleep. You are in a different physiological stage, but you may not have lost consciousness.

You usually are in this stage from minute 5 onto minute 10. And in stage two up to about 15-20 minutes. After that, you can fall into a deeper sleep; which waking up from is very problematic.

I used to set my alarm for 30 minutes. I'd way up, feel even more tired, and set my alarm for 30 minutes again. This was ruining my life.

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tl;dr

Even if you don't lose consciousness, you can still be 'sleeping' and a nap of 20 minutes will greatly help you feel healthy and well rested.

Certainly depends on how tired you are. I work at home, so I sleep around my own schedule as much as I want to and almost always feel rested. But when I am missing an hour or two of sleep (which a lot of people are, every damn day), I fall asleep in minutes given the opportunity in the middle of the day.

Not for everyone. For me, yes, at least 15 minutes, but my SO can often go out in less than 5. I never take naps for that reason, and she takes them frequently. Perhaps there's a correlation between the time it takes to fall asleep and the frequency of naps.

If it's that hard to fall asleep, then you don't need a nap. Problem solved. Depending on their job, diet, exercise, age, etc., some people just really really really really need a 10-15 minute nap in the early afternoon to get through the rest of the day. The need comes over you quickly, and you either do it or spend the rest of the day struggling to stay awake and focused on your work. I don't think anyone is suggesting that people who aren't actually tired take naps.

Depends on the person, so this isn't going to work for everyone. I can fall asleep in ~2 min. It's a curse when I need to stay awake.

I find just trying to sleep for twenty minutes can be rejuvenating enough for a powernap. I can't fall asleep on a whim either.

Especially since I think coffee for me is mostly a placebo. I know when I drink coffee I'm supposed to be more awake so there's no way I could take a nap after drinking one.