I've just read this:

http://www.runnersworld.com/running-tips/the-uncomfortable-truth-about-recovery

Interesting how much this fits my "stupid" theory about your body is just giving up. Let me explain my theory. I noticed that when I ramped my mileage to a previously unfathomable level, instead of getting extremely tired, sometimes, especially *before* rest days, I felt really good. It was like during the ramp-up, my body was screaming at me, begging to stop, but when I didn't, my body just "gave up". It didn't scream any more - with a bit of nagging pains and tiredness, it resolved to do the regimen. Then I took a rest day, and I was hardly able to get out of the bed the following day. Like my body thinking "oh, there *is* a chance of getting rest - give it to me now then!"

Of course I assumed this was just a stupid mind game - anthropomorphising my body - but now it seems like it actually may be true.