Sleep debt is the running total of hours your body needed but did not get. Skip an hour Monday and another two on Tuesday, and you are three hours down by midweek — and the deficit does not reset on its own. This calculator adds up the last seven nights against your target, gauges severity, and tells you how many weeks of catch-up sleep it will take to clear the balance.
The math is direct. For each of the last seven nights, the calculator subtracts hours slept from your nightly target. Nights at or above target count as zero — surplus sleep does not bank as credit, despite popular belief. The shortfalls add up to your total debt in hours.
Severity bands track average nightly deficit. Under one hour off-target averages out as none. One to two hours is mild. Two to three is moderate. Three or more is severe — that is the band where reaction time, mood, and immune function visibly degrade.
Recovery assumes you can add about one extra hour above your target each night, which is the rate the Van Dongen et al. 2003 chronic restriction study found subjects could realistically sustain. Total debt divided by seven gives weeks-to-clear. Anything past twelve weeks suggests behavioural change rather than recovery — that is when consistent earlier bedtimes matter more than catch-up sleep.
Targets adjust for age. Adults default to 7.5 hours. Teens need 8.5. Adults over 65 average closer to 7. Override the target if you know your personal sweet spot — most people who track sleep find it within thirty minutes of these defaults.
The headline number is your total shortfall across the last seven nights — every hour you slept below target, added together. It's a running balance, not a nightly snapshot, which is why one or two decent nights don't cancel out a run of bad ones.
The severity band translates that balance into how you'll actually feel. It tracks your average nightly deficit: under an hour is negligible; one to two hours is mild (concentration and mood dip); two to three is moderate (reaction time slows to roughly the level of mild alcohol impairment); three or more is severe — the band where immune, metabolic and accident-risk markers climb.
The weeks-to-clear figure assumes you can add about one extra hour above target each night — a sustainable rate, not a heroic twelve-hour weekend. If the estimate runs past twelve weeks, read it as a signal rather than a target: a deficit that large is a schedule problem, and the schedule is what needs to change.
The landmark study is Van Dongen and colleagues (2003), who held volunteers to 4, 6 or 8 hours in bed for two weeks. The 4- and 6-hour groups accumulated attention and working-memory deficits that worsened steadily day after day — a clear dose-response effect. By the end, the 6-hour group performed like people who had been awake for two nights straight.
The unsettling part: those same subjects rated their own sleepiness as only mildly elevated. Chronic short sleep quietly erodes your ability to notice you're impaired — which is exactly why a number on a screen is useful. The deficit is real even when you feel adapted to it.
And you can't simply repay it on the weekend. Depner and colleagues (2019) let sleep-restricted subjects sleep freely on weekends; they recovered some sleep, but their insulin sensitivity worsened and late-night snacking increased anyway — the metabolic cost didn't reverse. That's why this calculator counts nights above target as zero rather than as bankable credit.
For a recoverable debt (a few hours, under twelve weeks) the plan is unglamorous: add roughly an hour a night by going to bed earlier — not by sleeping in, which drifts your body clock later and makes Monday worse. Keep your wake time anchored even on weekends; a fixed wake time is the single strongest lever on the circadian system.
Time those earlier bedtimes to land at a sleep-cycle boundary, so you wake between cycles instead of mid-deep-sleep — the Sleep Cycle Calculator in the sidebar does that math. For severe debt, a short afternoon nap (20 minutes, before 3 pm) takes the edge off without stealing that night's sleep pressure.
If the debt keeps coming back — you clear it, then it's back a fortnight later — the problem isn't recovery, it's the routine feeding it. That's a behaviour fix: a consistent wind-down and a protected bedtime. The calming bedtime routine guide linked in the sidebar walks through the 30-minute version.
Mostly no. Studies show one or two long nights of recovery sleep do reduce same-week sleep pressure, but the cognitive and metabolic impairments from chronic short sleep do not fully reverse with weekend catch-up. The calculator treats surplus as zero because that is the conservative read of the evidence — pretending you banked credit will keep you from fixing the schedule.
Beyond twelve weeks, the recovery model breaks down. Sleep debt over that horizon almost always means a chronic schedule problem — shift work, parenting, untreated insomnia — and the fix is fixing the schedule, not catching up. If you hit the cap, the recommendations point you toward the underlying behaviour change.
The defaults match the National Sleep Foundation consensus ranges. If you wake naturally without an alarm after seven hours and feel sharp all day, your target is seven, not seven and a half. Override the target field. Anything below five or above eleven is unusual enough that the form refuses it — see a clinician if you are confident in those numbers.
Not for the total. The calculator sums deficits regardless of which night had what. The per-night breakdown is shown so you can spot patterns — for example, weekend recovery hiding weekday restriction. Enter oldest night first if you want the chart to read left-to-right chronologically.
Yes. Add nap minutes to the same-day overnight sleep, expressed in hours. A 20-minute nap is 0.33h. Naps under 30 minutes do help with daytime alertness but do not fully substitute for nighttime sleep — so the rolled-up number is approximate. For severe debt, the recommendation to schedule a short afternoon nap stands.
The bands track behavioural and physiological markers that emerge in chronic sleep restriction studies. Mild (1-2h average deficit) shows mood and concentration declines. Moderate (2-3h) shows reaction-time impairment comparable to mild alcohol intoxication. Severe (3h+) shows the immune, metabolic, and accident-risk markers that drive the public-health concern around sleep loss.
Pair with debt math — when you're behind, bedtime moves earlier.
A 90-min nap can recover ~60 min of debt — often the fastest stopgap.
See which of the 5 factors is feeding your debt.
Caffeine timing fragments deep sleep and accelerates debt accrual.
Tracks debt automatically once you wear it. Independent verdict.
Recovery framing maps cleanly onto the debt model.