The law of diminishing returns and the bell curve

We all know this. More midnight oil == Increasing difficulty in your books.
But is there a point where increased hours makes you forget what you have revised for each additional hour?

Luckily no, but there was a story (email, possibly fabricated) of a young woodcutter who joined a group of older woodcutters. He worked hard but was disturbed that, despite his greater energy expended, he was cutting less wood. Where else the older men actually took tea breaks.
Only when he finally approached them did he know the answer, that they older men took the time to rest and sharpen their axes. Thus they do more with less effort.

-eof