Sleep Debt

Sleep debt is the accumulated difference between the sleep you need and the sleep you got. Aim for eight hours, sleep seven, and you are an hour down; do that four nights running and you are four hours down.

The idea is useful because the effects of short sleep accumulate in a way that a single night's figure hides. Research on restricted sleep finds that people adapt to feeling tired long before they adapt to performing badly, so a week into a deficit you will usually rate yourself as fine while your reaction times say otherwise. A running total is harder to talk yourself out of.

How much sleep you need is personal. Seven to nine hours covers most adults, teenagers need more, and the practical test is whether you wake without an alarm feeling rested rather than whether you hit a number.

What it is not

It is not a balance you can settle hour for hour at the weekend. A long Sunday restores alertness quickly and some of the other effects much more slowly, so a consistent bedtime does far more for the total than a weekend lie-in does.

Where you see it in FitnessSyncer

Set your target sleep in your preferences and your sleep debt is worked out from every night that arrives from a connected tracker, charted on your Dashboard and in the built-in Sleep report. Our sleep debt calculator works out a single night by hand.