In times where I know I have been over-doing it, I have found myself very tired, yet strangely unable to sleep properly and your performance will deteriorate.
I always try to alternate hard rides with slow "recovery" rides and it's probably a good idea to have a rest day. Don't head out on a fast, hilly 50 mile ride if you feel tired and all done out before you leave. Listen to your body.
I'm sure someone will have proper medical knowledge.