This brings up a key concept, namely the inverted U. The complete absence of stress is aversively boring. Moderate, transient stress is wonderful-various aspects of brain function are enhanced; glucocorticoid levels in that range enhance dopamine release; rats work at pressing levers in order to be infused with just the right amount of glucocorticoids. And as stress becomes more severe and prolonged, those good effects disappear {with, of course, dramatic individual differences as to where the transition from stress as stimulatory to overstimulatory occurs; one person's nightmare is another's hobby}.
( Robert M. Sapolsky )
[ Behave: The Biology of Humans ]
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