Sleep debt
Pajdo's estimate of how far your sleep fell short of your personal sleep need over the last 7 nights, with long nights paying back short ones.
First, Pajdo works out your sleep need: your median night over the last 60 days in Apple Health, naps included, kept between 7 and 9 hours. He needs 14 nights to start. That way a habit of short sleep still gets measured against 7 hours.
Then he compares each of the last 7 nights with that need. A long night pays back a short one, the total never drops below zero and nights without data add nothing. It's an estimate, since there's no validated formula for tracking sleep debt at home, and real recovery is messier. In one lab study, people who'd spent 5 or 7 hours in bed for a week still reacted more slowly after 3 nights of 8 hours.
Short sleep adds up quietly. In another lab study, adults limited to 6 hours in bed for two weeks slipped on attention tests about as much as after a full night without sleep, yet felt only a bit sleepy.
Pajdo brings it up from about 5 hours of debt, counted over at least 5 nights.
What tends to push it up
- Nights shorter than your sleep need
What tends to pull it down
- Longer nights later in the same week
Keep reading
Sources
- The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation, Sleep (Van Dongen et al., 2003)
- Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study, Journal of Sleep Research (Belenky et al., 2003)
- Recommended amount of sleep for a healthy adult: a joint consensus statement, American Academy of Sleep Medicine and Sleep Research Society (J Clin Sleep Med, 2015)
- National Sleep Foundation's updated sleep duration recommendations: final report, National Sleep Foundation (Sleep Health, 2015)
Checked against these sources on September 15, 2026.
This is general wellness information, not medical advice. If a number worries you, or stays away from your usual for a while, talk to a doctor.