Coverage shape
Three-shift or twelve-hour rotations, around the clock, on a ward that cannot simply run short the way an office can absorb an empty desk for a day.
Qualifications
Nursing registration itself, plus specialty endorsements, plus mandatory training that must be refreshed on a fixed cycle. Each behaves like a licence with an expiry, not a one-time check completed at hiring.
Statutory rest
A minimum rest between shifts and a cap on consecutive nights are common across health systems. The specific limits vary enormously by country and by collective agreement, so none are stated here.
Demand pattern
Acuity, not just headcount, drives how many staff a ward needs on a given shift, and that figure can move within the same day as patients are admitted or discharged.