Sleep Deprivation Calculator: How Impaired Are You After Hours Awake?

After about 17 hours awake, people in lab studies performed about as poorly as they did with a blood alcohol level of 0.05%, and after 24 hours about as poorly as at 0.10%, which is over the US driving limit ^[1]. Wake up at 7 AM and that 17-hour mark arrives at midnight. Enter when you woke and how long you slept to see where you stand. And whatever the number says: if you feel drowsy, do not drive.

How the calculator works

Two different things wear you down, so the calculator looks at both: how long you have been awake today, and how much you slept before that.

Hours awake is plain subtraction, the time you choose minus the time you woke up. If that time falls after midnight, pick "the next day" and it counts on across midnight. Those hours then land in one of four bands. Lapses of attention start to pile up once you pass roughly 16 hours awake ^[5]. At 17 hours the lab comparison is about 0.05% blood alcohol ^[1][2], and at 24 hours about 0.10% ^[1]. Below 16 hours you are under the point where the studies found alcohol-level impairment, which is not the same as being unimpaired.

Crash odds come from how much you slept in the 24 hours before. In a US study of 6,845 drivers involved in crashes, those who had slept 6, 5 or 4 hours had about 1.3, 1.9 and 2.9 times the odds of having caused their crash, compared with drivers who had slept 7 to 9 hours. Under 4 hours, the odds were about 15 times higher ^[4]. The calculator also flags the hours between midnight and 6 AM, when your body clock adds its own push toward sleep and drowsy-driving crashes are most common ^[8].

Where the alcohol comparison comes from

In 1997, two Australian researchers tested volunteers through a long stretch without sleep, then compared the results with how the same people did after measured doses of alcohol. That let them express tiredness in a unit everyone understands. Seventeen hours awake looked like 0.05%, and 24 hours looked like 0.10% ^[1].

A follow-up with 30 transport workers and 9 army staff found much the same thing. After 17 to 19 hours without sleep, which for them meant about 10:30 PM to 1 AM, performance on some tests matched or fell below the 0.05% level, and response speeds on some tests were up to 50% slower ^[2]. US public health training for nurses uses the same 17-hour and 24-hour figures and points out that driving skills already suffer at 0.05%, even though the US legal limit is 0.08% ^[3].

Example: a 7 AM start and a long night

You woke at 7:00 AM after a normal 7-hour night. Here is how the evening plays out:

Now rewind and say you only slept 4 and a half hours before that 7 AM alarm. The hours awake don't change, but the crash odds from short sleep climb to about 2.9 times baseline ^[4], and the two problems stack on top of each other.

Why you can't judge it from the inside

Sleepiness is sneaky. When you are short on sleep you can slip into microsleeps, brief moments of sleep that you can't control and might not even notice ^[7]. Ever pulled into your driveway with no memory of the last stretch of road? That may have been one.

Weeks of short nights make it worse. In a two-week lab study, people held to 6 hours or less in bed a night built up attention deficits as bad as up to two full nights without sleep, yet their own sleepiness ratings barely budged after the first few days ^[5]. That is why the calculator asks about the past two weeks, and if short nights are your normal, the sleep debt calculator shows how much you are running behind.

What helps and what doesn't

Sleep is the only real fix. If you are already driving and feel tired, the CDC's advice is simple: get off the road and rest. Turning up the radio or opening the window has not been found to help ^[6]. Caffeine can perk you up for a few hours, but the sleepiness returns when it wears off ^[8]. Some people pair coffee with a short nap, the caffeine nap, but nothing you drink undoes a night without sleep. In one CDC survey across 19 states, about 4% of adults said they had fallen asleep at the wheel at least once in the previous 30 days ^[6].

Facing a short night by choice? Our guide on whether to sleep 2 hours or stay up covers that decision. Already past it? See how to recover from an all-nighter.

Limits and assumptions

This is not a fitness-to-drive test. The figures are group averages from small lab studies and a crash study. The calculator can never tell you that you are fit or safe to drive. If you feel drowsy, do not drive.

People also vary. Some are far more sensitive to lost sleep than others, and the time of day matters as well as the hours. The bands assume a daytime schedule, so if you work nights your body clock may not match the clock on the wall and the result is only a rough guide. Alcohol makes everything worse: even a small drink on top of a long day pushes performance down further than either alone ^[3].

If you nod off without meaning to, feel very sleepy despite enough time in bed, or snore loudly with pauses in your breathing, see a doctor and ask about a sleep disorder.