depletion zone
noun phrase
A period during which one has pushed beyond one’s known safe range of expenditure of energy or deprivation of sleep, food, or drink, and it is not known how much further one can go before disaster will strike.
Stanislav had yet to collapse on the job, but he had pushed into the depletion zone on several uncomfortable occasions. It always came after several days of much stress and little sleep, and he could tell he was entering it when he found himself suffering from spontaneous waves of dizziness throughout the day.