Crying Yourself to Sleep

I never understood how one can cry oneself to sleep.  I don't know about everyone else, but when I cry, my nose gets all stuffed up and then it's incredibly difficult to breathe.  And when I can't breathe, I find that I also cannot sleep very well.  It's kind of like when I'm sick and you have a stuffy nose... it's hard to sleep.  So what's the deal with crying yourself to sleep?  I guess it's just an expression... for crying right before you go to sleep and then falling asleep from exhaustion after you stop crying and can breathe again.