LodestarSleep debt is the gap between the sleep you need and the sleep you get, added up over days. A few short nights don't feel like much, but they compound.
How much sleep do you need?
For most adults, the answer is at least seven hours a night, according to a consensus statement from sleep researchers (Watson and colleagues, Sleep, 2015). Some people need more. Your own need also goes up after hard training, during illness, and when you're already short.
Why it adds up faster than it feels
In a well-known lab study, people who slept six hours a night for two weeks performed about as badly on attention tests as people kept awake for a night or two, yet they rated themselves only slightly sleepy (Van Dongen and colleagues, Sleep, 2003). Sleep debt builds quietly, and we're bad at noticing it.
Paying it back
- A weekend lie-in helps, but not completely. Research on recovery sleep found that catching up at the weekend didn't undo all the effects of short weekday sleep (Depner and colleagues, Current Biology, 2019).
- Spread it out. Going to bed 30 to 60 minutes earlier for several nights is easier on your body clock than one very long sleep.
- Naps count a little. A short early-afternoon nap can take the edge off without hurting your night.
- Keep your wake time steady. It anchors your rhythm while you catch up.
Tracking it with your Fitbit
Your Fitbit records how long you actually slept each night, which is all you need to track debt. The tricky parts are knowing your own need and weighting recent nights more than old ones.
Lodestar does this for you on iPhone: it estimates your sleep need, tracks the debt you're carrying from recent nights, and suggests how much sleep tonight would help you catch up. It's all worked out on your phone from your Fitbit data.