The ledger you cannot feel
Two weeks of six-hour nights produce vigilance performance equal to a full night awake — while subjects rate themselves as mildly tired, not impaired. Sleep debt is the rare deficit that disables its own alarm system, which is why objective accounting (the debt calculator) beats the subjective "I'm fine."
The repayment mechanics
Recovery sleep preferentially restores deep and REM stages, so repayment is efficient: +60–90 minutes nightly for a week, or two recovery mornings within two hours of your usual wake time. What fails: the 12-hour Sunday (partial mood reset, full rhythm disruption) and the belief that debt can wait for a vacation.
Prevention is the protocol
Debt is cheapest to avoid: a consistent need-based bedtime, a caffeine curfew, and a bedroom that is dark, cool, and boring. The debt calculator is the audit; the bedtime calculator is the budget. Run both monthly and the dramatic repayments become unnecessary — which is the point.