Free tool
Sleep debt calculator
Sleep debt accumulates faster than it repays. Enter your last seven nights to see where your balance actually stands, and how long a realistic repayment plan would take.
Hours actually slept, last seven nights
Running sleep debt
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How this calculation works
For each night, the calculator takes the difference between your stated nightly need and the sleep you actually got. Nights below your need add their full shortfall to the balance. Nights above your need subtract at half rate.
That asymmetry is deliberate. Extra sleep genuinely helps repay a deficit, but the research does not support a one-for-one refund: several measures tied to metabolic and cognitive function do not fully normalise after a single long night. Treating surplus as a half-value credit is a reasonable, slightly conservative model.
The balance is also floored at zero. You cannot bank sleep in advance in any meaningful way, so a run of long nights does not build credit against future deprivation.
This is the same model the Apex Health: Recovery Coach sleep ledger uses, except that the app reads your actual sleep from Apple Health across fourteen nights and adds a circadian regularity score from when you went to bed and woke.
Finding your real nightly need
The default of eight hours is a reasonable starting point, but sleep need is individual and largely genetic. Most adults fall between seven and nine hours.
The practical way to find yours: on a stretch of days with no alarm and no significant existing debt, such as a week of holiday after the first few catch-up nights, most people converge on a fairly stable duration. That is your number. It is usually longer than the amount you have trained yourself to tolerate on work days.
Be honest with the input. Setting your need to six hours because that is what you usually get will produce a comfortable answer and a useless one. See our guide to sleep debt for more.
Common questions
How is sleep debt calculated?
Sleep debt is the running total of the difference between your nightly sleep need and the sleep you actually obtained, accumulated over a rolling window, commonly seven or fourteen nights. Nights below your need add their shortfall in full. Nights above your need repay some of the balance, though most models credit surplus at less than full value because recovery sleep does not undo prior loss hour for hour.
How long does it take to recover from sleep debt?
For a modest deficit of two to four hours, several nights of sleeping 30 to 60 minutes beyond your need will usually clear it. Larger deficits take proportionally longer, typically one to two weeks of consistent adequate sleep. A single extended night restores alertness substantially but does not fully resolve the deficit.
Is 6 hours of sleep enough?
For the large majority of adults, no. Population data consistently place adult sleep need between seven and nine hours, and the proportion of people who genuinely function well on six hours or fewer is very small, far smaller than the proportion who believe they do. Chronic restriction to six hours produces measurable declines in attention and metabolic function that people typically fail to notice in themselves.
Does this calculator store my data?
No. The entire calculation runs as JavaScript inside your browser. The hours you type are never transmitted, never stored, and never seen by us. You can confirm this by loading the page and then disconnecting from the internet; the calculator will keep working.
Wake up to a verdict, not a spreadsheet.
Apex Health: Recovery Coach turns the Apple Health data you already generate into one clear decision each morning. Free, private, and entirely on your device.
Apex Health: Recovery Coach provides general wellness and fitness information computed from your own data. It is not a medical device and does not diagnose, treat, or prevent any condition. Always consult a qualified professional for medical concerns.