For me last summer my periods of extended rest were just brief moments to partially recover. I don't think I fully recovered until some extended leave in late autumn. My wife will tell you that on the off periods I just wanted to sit quietly and not engage in life as I was too tired. Whilst I was doing this I was having to manage my sleep periods so that I could go back to the next run of earlies. I was no better than an automaton.
Fatigue is cummulative and I disagree that fatigue only occurs at the end of a run. I think I was most fatigued recently on day 3 of 5, but in the middle of a very intense block of almost ninety hours in a three week period.