Maybe this is a good time to do something I haven't done in a while: Conduct a poll. You will find it in the top right-hand corner of this blog. The question is: What do you do on days when your body just doesn't feel like practicing? Please vote!
If your answer happens to be "drink some coffee"--actually, even if it isn't--you should also check out the following recent video by Kiki warning about the consequences of excessive coffee consumption on the adrenal glands. I think Matthew Sweeney said what amounted to pretty much the same thing in his book Astanga Yoga As It Is, when he warned that using coffee to wake up to practice is only a stop-gap measure, and ultimately doesn't address the root cause: Whatever it is that is making you tired.
Don't get me wrong: I'm not telling anybody to give up coffee (I love espresso too). But all this is definitely something to be mindful of.