Paul Briginshaw/Millennium Images, UKWhats the difference between your time spent in bed and your bank balance? No, this isnt the start of a terrible joke and the answer is less than you might think.We all have the odd occasion when we stay up too late and dont sleep enough. Think of this as the equivalent of splurging on an expensive dinner: you probably shouldnt have, but your bank balance hopefully wont suffer too much.This article is part of special series investigating key questions about sleep. Read more here.But regularly going without enough sleep a problem for many people, with the US Centers for Disease Control reporting that a third of adults there get less than 7 hours a night could have you racking up a sleep debt, with real consequences for physical and mental health (see Why your chronotype is key to figuring out how much sleep you need). Like paying back a financial debt, catching up on sleep takes planning.Part of the problem is that we might not know how much sleep debt we have accrued and how badly it is affecting us. In one study, for instance, participants were randomly selected to get 4, 6 or 8 hours per night for 14 days straight. By the end, those getting 6 hours or less exhibited a cognitive deficit equal to missing up to two entire nights of sleep. However, despite feeling worse after a couple of days, from then on the restricted sleepers didnt necessarily notice their cognitive abilities continuing to decline. The tired brain cant detect how tired it is, says Russell Foster, a