The title says it all:

Why is "I'm stressed and need the day off" a bad reason to take the day off? And how can i phrase it so that it would be more acceptable?

If I'm so stressed or tired that I'm making mistakes, how do I tell my boss this without seeming like a slacker? How do I get him to understand this will not go away with a single night of good sleep?

I'm sorry if these are multiple questions, but again, I'm really stressed right now and I'm having trouble thinking straight.