Sleeping 4 to 5 Hours Before Driving Carried 4.3 Times the Crash Odds in a 7,234-Driver Study

Of the 7,234 drivers in a federal crash-investigation sample, 80 told investigators they had slept less than four hours in the previous 24. Seventy-four of those 80 were judged to have contributed to the crash they were in. Among the 6,308 drivers who had slept seven hours or more, the count was 3,026, or 48 percent.

In the AAA Foundation for Traffic Safety’s December 2016 report by Brian Tefft, drivers who had slept 6:00 to 6:59 in the previous 24 hours had 1.3 times the odds of contributing to their crash compared with drivers who had slept seven hours or more. The odds ratio was 1.9 at 5:00 to 5:59, 4.3 at 4:00 to 4:59, and 11.5 under four hours. The analysis covered 4,571 crashes between July 2005 and December 2007 that occurred between 6 a.m. and 11:59 p.m.

Sorting drivers into cases and controls

The data came from the National Highway Traffic Safety Administration’s National Motor Vehicle Crash Causation Survey. Each crash involved at least one car, pickup, van, minivan or SUV towed from the scene, with emergency medical services dispatched. Specially trained investigators interviewed the drivers, and one question was the start and end time of every stretch of sleep in the preceding 24 hours, naps longer than 30 minutes included.

A driver counted as having contributed to the crash if investigators assigned the “critical reason” to that driver and the reason was an unsafe or illegal action, an inaction, or an error. Falling asleep at the wheel qualified. The report says this coding was “neither informed by nor intended to inform assessment of ‘fault’ for legal or insurance purposes.”

Those drivers became the cases: 3,579 of them. The controls were 3,655 drivers who were in the same kinds of crashes but were not assigned the critical reason. The logic is that someone struck by another motorist is a fair sample of ordinary drivers on the road that day, so the sleep they report shows how rested drivers in general are. The report adjusted for age, relation to intersection, time of day and any recent change in sleep schedule. It dropped drivers whose critical reason was unknown (260), who had a heart attack or stroke just before the crash (138), or who had no sleep data (2,573, of whom 77 percent were never interviewed).

The odds, band by band

Sleep in the prior 24 hours	Drivers	Contributed to crash	Adjusted odds ratio (95% CI)
7:00 or more	6,308	3,026 (48.0%)	1.0 (reference)
6:00 to 6:59	617	331 (53.6%)	1.3 (1.1 to 1.7)
5:00 to 5:59	158	99 (62.7%)	1.9 (1.3 to 2.6)
4:00 to 4:59	71	49 (69.0%)	4.3 (2.2 to 8.3)
Under 4:00	80	74 (92.5%)	11.5 (2.9 to 45.7)

Counts are from the report’s Table 1. The percentages in the third column are my own division of unweighted counts; the odds ratios are the report’s weighted, adjusted estimates.

The shape is not a straight line. Dropping from seven hours to six-plus lifts the odds from 1.0 to 1.3. Dropping to five-plus takes them to 1.9. Below five, they break upward. The confidence interval on the bottom row, 2.9 to 45.7, is wide because only six of the control drivers reported under four hours, so 11.5 is better read as “far higher than the rest” than as a precise multiple.

The alcohol comparison, and its limits

The report states that the estimated rate ratio for crash involvement after only 4 to 5 hours of sleep, compared with 7 hours or more, “is similar to the U.S. government’s estimates of the risk associated with driving with a blood alcohol concentration equal to or slightly above the legal limit,” and that the increase for under four hours “is much greater.” That is a comparison between two separate bodies of work, with different designs and different populations. It gives a sense of scale. It does not mean four hours of sleep and a blood alcohol reading of 0.08 impair a driver in the same way.

Reading the table by hand: where the extra crashes sit

The tallest odds ratio is not where most of the extra risk sits.

Population-attributable arithmetic multiplies each band’s share of ordinary drivers by how far its odds exceed 1. The weighted shares among control drivers in Table 1 are 0.2 percent for under four hours, 0.4 percent for 4:00 to 4:59, 0.9 percent for 5:00 to 5:59 and 7.2 percent for 6:00 to 6:59. Multiplying by (odds ratio minus 1) gives 2.1, 1.3, 0.8 and 2.2, a total of about 6.4. Dividing that by one plus the sum as a proportion (1.064) gives about 6 percent, which would be the share of crash involvement among these drivers tied to having slept under seven hours.

By that division, the 6:00 to 6:59 band and the under-four band each carry about a third of the excess. The 4-to-5-hour band carries about a fifth and the 5-to-6-hour band about an eighth. A modest per-driver increase, applied to the large share of drivers who are one night short of seven hours, adds up to about as much as the dramatic increase applied to a tiny group.

This is a back-of-envelope reading and the study does not make the claim itself. The under-four share rests on six control drivers, so its third could plausibly be a fifth or a half. The calculation also covers only the 6 a.m. to midnight window. A rough match with the AAA Foundation’s own earlier estimate, as many as 7 percent of towed-vehicle crashes involving a drowsy driver (Tefft, 2012 and 2014), comes from a different dataset and method, so it is a plausibility check and nothing firmer.

Shortfall against your own usual night

The report ran a second analysis keyed to each driver’s reported usual sleep. Drivers who usually slept 4 to 5 hours had 5.4 times the odds of contributing to a crash (95% CI 1.6 to 17.6) compared with those who usually slept seven or more. The usual-sleep estimates for under 4, 5 to 6 and 6 to 7 hours were not statistically significant.

Relative to each driver’s own norm, the last night’s shortfall mattered once it grew past an hour:

Slept this much less than usual	Adjusted odds ratio (95% CI)
Up to 1 hour	1.2 (0.9 to 1.6), not significant
1 to 2 hours	1.3 (1.0 to 1.7)
2 to 3 hours	3.0 (2.0 to 4.3)
3 to 4 hours	2.1 (1.0 to 4.4)
4 hours or more	10.2 (4.6 to 22.9)

The dip from 3.0 to 2.1 sits inside overlapping intervals and should not be read as a real reversal. The pattern to take from the table is the jump after two hours of shortfall.

What the midnight-to-6 a.m. gap hides

Tefft calls the missing hours between midnight and 6 a.m. “possibly the most significant limitation.” The crash survey did not collect those hours at all. Two checks inside the data hint at what was missed. Drivers who crashed in the first and last hour of coverage, 6:00 to 6:59 a.m. and 11:00 to 11:59 p.m., had slept an average of 7 hours 16 minutes in the prior day, against 8 hours 16 minutes for crashes at other times. And 8.0 percent of drivers in those two hours reported under five hours, against 1.9 percent in the rest of the day (P = 0.005). The report concludes that it may have underestimated both the prevalence and the risk of driving short on sleep.

A second oddity: only 3.7 percent of drivers in the sample (the weighted share across all sampled drivers) reported under six hours of sleep, compared with 9 percent of drivers in the American Time Use Survey. The report offers two explanations. Short sleepers may simply drive less during the day. Or their trips may cluster in the overnight hours the sample leaves out.

Sleep was self-reported, which has its own track record of error; a 2008 comparison of diaries and wrist sensors found people overstating their sleep by about 0.8 hours. Fewer than 4 percent of drivers were tested for alcohol, so drink and drugs could not be separated from sleep.

The morning drive

The study’s tally counted hours slept, not hours in bed, and it counted naps over 30 minutes. A driver in bed from 11:30 p.m. who is out of the house at 6:30 a.m. after about 6.5 hours of actual sleep sits in the 6:00 to 6:59 band. The report did not test whether a mid-day nap removes the risk, only that naps were included in each driver’s total.

Professional drivers are the one group for whom the rest question has been written into rules. The FMCSA’s drive-time and restart limits are a regulator’s answer to a version of the same problem. For everyone else, the closest the report comes to a rule is the pair of consensus lines it cites: at least seven hours a night for healthy adults aged 18 to 60 (Watson et al., 2015), and no driving after two hours or less in a day (Czeisler et al., 2016).

The reference group is also a pooled bin of everyone at seven hours or more, so the report cannot say whether eight hours is safer than seven. Within the range it tested, the bins below seven get worse, and the lowest get much worse.