The need to rest and recover from a workout is key in driving performance. There probably isn't really any over training. There is, more likely , under recovery.

The key then, is, before you are exhausted, how do you know when to rest. Is it possible that heart rate variability is the key to planning and taking rests.

In simple terms, everybody knows about heart beats. However what is probably news to most is that the space between the beats can vary. The more consistent the gap between the beats, weirdly, the more under recovered you are. The more variability, the better you are! see it like this. imagine an exhausted stressed person walking down the street. They probably plod along at the same pace. The person full of life is probably skipping along, moving slightly erratically, or with more "variability".

HRV  could put Biofeedback back in the frame. I short where techniques are learned to control stress, heart rate and a host of other variable. The efficacy of which has arguably been established by studies such as van der Zwan's Physical activity, mindfulness meditation, or heart rate variability biofeedback for stress reduction: a randomized controlled trial. Click here for abstract.

However, it has long been accepted that breath control has a value in mental calmness  and those with even a basic heart rate monitor can see the effect that 4 x4 breathing can have on a racing heart ( breath in for 4 secs, hold for 4, out for 4. relax for 4, breath in for 4). Now its possible to see if these practices can effect heart rate variability and promote athletic recovery