Sleep debt does not work like a financial one, where a large repayment settles the account. After several short nights, a single long sleep restores alertness only partially; measures of reaction time and judgment stay depressed for days. Worse, the impairment is difficult to notice from the inside. People who have slept five hours a night for a week rate their own alertness as nearly normal while performing measurably worse, which makes self-assessment an unreliable guide and is the reason safety-critical industries set hour limits rather than asking workers whether they feel fit.