I'm the same. These guys say 20 minutes is the best, but in that case I think we need to choose a better term than "nap", because I won't be asleep for almost any of that 20 minutes. Rest, perhaps.
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.
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.