John Broz, the guy who runs Average Broz Gymnasium, says that feeling run down / bad after heavy training doesn't mean anything bad. According to him, serotonin is the neurotransmitter responsible for muscle contraction, and when you "lift heavy" your body has to adapt by temporarily shutting down the serotonin production faculties in your brain so that it can renovate them for higher performance. This is why you feel bad / "overtrained" he says (serotonin levels also control mood). But in reality, you are just adapting to a higher stress and "getting stronger." He has his lifters doing squats to max six or seven days a week, apparently.
The whole post where he talks about this seems suspicious to me but it's an interesting argument because it goes against our instincts. Instead of laying off when we feel bad, he says we should just keep "pounding away."
