The running total of sleep you’ve missed versus what you need.
Sleep debt is the cumulative gap between the sleep you need and the sleep you actually get. Miss an hour a night for a week and you’re carrying roughly a seven-hour debt. It’s real and it compounds — affecting mood, focus, judgment, and even how anxious you feel. You can pay some of it back, but not all in one lie-in: recovery comes from a sustained return to adequate nightly sleep, not a single marathon.