You'd be surprised. I've been aiming for 20 minutes assisted by the iPod's Timer app, and I've hit 11-minute sessions that should qualify as 'naps'.

I've noticed that if it accidentally turns into a longer session, a sweet snack and a glass of water helps with the irritability a little.

I thought I was one of those people too... every time I try to nap it seems to end up with me just lying down and trying to fall asleep for 20 minutes. However, after moving in with my girlfriend she has confirmed that I do indeed manage to fall totally asleep during these attempted naps.

I did some looking around and apparently earlier sleep stages can easily be confused for being awake, especially for light sleepers. It may sound weird, but sleep perception is a funny thing. Some people nap quite easily, yet conclude they did not sleep at all because they never perceive themselves falling asleep or waking up.