Track accumulated sleep debt.
Sleep debt is the nightly shortfall multiplied by the nights it has persisted, and it accumulates faster than most people expect: ninety minutes short across a fortnight is a full night's sleep owed. Repayment is limited by how much extra sleep you can actually take in one night — usually an hour or two — so clearing a large debt takes a week or more of longer nights rather than one weekend.
Sleep debt accumulation
Debt = (sleep needed - sleep achieved) x nights; recovery nights = debt / extra sleep available per night
Chronic sleep loss is associated with impaired alertness and health risks, and excessive sleepiness can indicate an underlying sleep disorder. Do not drive when sleep deprived, and seek medical advice for persistent problems.
Partially and gradually. Recovery sleep restores much of the performance deficit, but a large accumulated debt takes several nights of extended sleep rather than one long lie-in.
Most adults need seven to nine hours. A reliable test is how long you sleep on a holiday week with no alarm once the initial catch-up has passed.
It helps but rarely clears a week's debt, and a large weekend shift also delays your body clock, which makes the following week harder.