What do you do when you feel like crap?

For the last four days I've felt like crap. Let's call a spade a spade.
I can barely muster the energy to type in my own name (and I've misspelled it more than once today). What do you do when you're not 100 percent? Do you call in sick, only to have that day's work added to your daily duties? Or do you try to tough it out and go into work, only to realize you're not at your most creative?
I'm usually in the latter group. I don't think weariness is contagious and I'd better snap out of it before Thursday, my deadline day. Maybe I can become instantly brilliant in one day and turn my ordinary prose into masterpieces of the written word. Don't laugh too loudly, it could happen.