Sleep Debt Is Not a Bank Account
You cannot repay it on Saturday, and the metaphor is doing real damage to how people plan their weeks.
The banking metaphor is comforting and mostly wrong. It implies a ledger — five hours short across the week, sleep five extra on Sunday, balance restored.
Recovery does not work like that, and believing it does leads people to design weeks that never recover.
What recovers and what does not
Some functions rebound quickly. Subjective alertness returns close to baseline after one or two long nights, which is precisely the problem — you feel recovered well before you are.
Glucose metabolism and immune markers take considerably longer. Studies of recovery sleep after a week of restriction find some measures still impaired after two nights of extended sleep, even as participants report feeling fine.
The gap between feeling and function
This is the part that matters practically. Self-assessed alertness is a poor proxy for performance under restriction. People who are measurably impaired routinely report feeling normal, and the impairment tends to be largest on tasks requiring sustained attention.
You lose the ability to notice, which means you cannot use how you feel to decide whether Saturday was enough.
What to do instead
Treat sleep as a constraint rather than a variable. Decide the hours first and fit the week around them.
This sounds like advice nobody can follow, and for some weeks it is. But most chronic short sleep I see is not caused by an immovable obligation. It is caused by treating sleep as the flexible item because it is the only one that does not complain when you take from it.