Sleep debt

The arithmetic is the whole idea. The AASM and the Sleep Research Society put adult sleep need at seven or more hours a night for most people aged 18 to 60. Come up forty five minutes short from Monday to Friday and you have lost close to four hours by the weekend, without a single night that felt dramatic. That is the character of sleep debt: it is built out of nights that seemed fine.

The unnerving part is that the subjective signal fades. After several short nights people commonly rate their own alertness as roughly stable while objective performance keeps dropping. You adapt to feeling worse, not to functioning better, and the gap between how sharp you feel and how sharp you are widens quietly.

What paying it back looks like

Recovery is real but it is not one for one, and it is not instant. A single enormous weekend lie in gives back some of what was lost, and simultaneously drags your body clock later, which is exactly how social jetlag is manufactured. You wake up on Monday having repaid some debt and taken out a new loan against your schedule.

The version that works is duller. Add thirty to sixty minutes to the front of the night, keep your wake time roughly where it is, and hold that for several nights running. Debt paid at the bedtime end does not move the clock the way debt paid at the wake time end does. If you want a wake time you can actually keep, the best time to wake up works through how to choose one.

Two things debt is not. It is not the same as sleep pressure, which is the drive built up since you last slept and is cleared by one good night. And it is not a diagnosis: if you are sleeping enough hours and still exhausted, that is a reason to see a doctor rather than to sleep more. Tock tracks sleep debt alongside total sleep time in its insights, which mostly saves you doing the sums, and it is not a medical device.