I'm just like you, @annadragon - if I try to take a nap I'll be out for hours. If I can even fall asleep, that is. I usually can't. And if I do, then I'll stay up too late, sleep late, need a nap, etc.
I've concluded that napping is just not for me. Luckily, it's pretty easy for me to get 8 - 10 hours every night, so napping is not necessary.