Naps

I was considering a cat nap. My neck hurts and a couple aspirin combined with rest may help relieve the pain. Then in its pain influenced fog, my mind began to wander. Why call it a "cat nap"? My own cats sleep, on average, about 25 hours per day. No sleep they get even resembles a nap. They basically are unconscious most of the time with intermittent, short bursts of wakefulness. I wouldn't be surprised to find one of them asleep with her face in the food bowl. So how can a cat nap be a short term sleep?  Wouldn't a cat nap be more akin to a coma?  

Also, why do we say that we want to take a nap?  From whom or what do we take it?  It's like the man who says, delicately choosing his language, that he has to take a pee.  Really?  Doesn't he deposit rather than take?

How do foreigners ever understand English?  Idioms are another problem, but the phrases "cat nap" and "take a nap" are just odd.  My guess is that these expressions are mistakes that through repetition have become part of the lexicon--like using impact as a verb.