Drowsy driving: an underestimated risk

Drowsy driving remains one of the road risks most poorly assessed by drivers themselves. Official figures nonetheless make it one of the leading causes of fatal accidents, particularly on highways.

It is also very widespread. In the 2026 survey by the Institut national du sommeil et de la vigilance (INSV) and the VINCI Autoroutes Foundation, one driver in four reported having experienced at least one episode of drowsy driving in the past year. The proportion reached 43% among people who work nights or shifted schedules. In the United States, a CDC survey showed that about one adult in 25 admitted having dozed off while driving at least once in the previous month.

A major factor in fatal accidents

On French highways, fatigue and drowsiness are involved in about one fatal accident in five, according to 2025 figures from the Association des sociétés françaises d'autoroutes (ASFA, the French highway operators' association). They were long the leading cause of death on this network, with up to one fatal accident in three depending on the year. They remain today among the top factors, alongside alcohol, drugs, and speed. The monotony of highway driving particularly favors gradually falling asleep, often without the driver being fully aware of it: no intersections or traffic lights, long and understimulating trips.

In the United States, official federal road safety statistics (NHTSA) record about 600 to 800 deaths a year linked to drowsiness. This figure is based on police reports, and every specialist considers it a significant undercount: unlike alcohol, no test can detect drowsiness after a crash. In 2024, the AAA Foundation for Traffic Safety reconstructed this figure using in-depth surveys. It estimates that nearly 18% of fatal crashes between 2017 and 2021 involved a drowsy driver, or nearly 30,000 deaths over five years. At the global level, the WHO counts about 1.19 million road deaths each year. Drowsiness isn't tracked as a dedicated statistic there, but international studies generally attribute 10% to 20% of serious accidents to it.

Specific high-risk time windows

The risk isn't spread evenly across the day. Drowsiness-related accidents cluster around two time windows: the late night and early morning, roughly between 2am and 6am, and the early afternoon, between 2pm and 4pm. These periods correspond to when natural alertness is at its lowest, tied to the circadian rhythm. British researchers Jim Horne and Louise Reyner had already shown this in 1995 by analyzing accidents on UK motorways and main roads.

The late-night risk is often underestimated during major holiday departures. Leaving at 3am “to avoid traffic” means driving at the exact moment the internal clock pushes most strongly toward sleep, often after a night cut short by packing and preparations. It's better to leave after a proper night's sleep, even if it means driving a bit longer.

The equivalent of drunk driving

One figure helps put the scale of the risk into concrete terms. Staying awake for 17 consecutive hours causes a drop in performance comparable to that caused by a blood alcohol level of 0.5 g/L, the legal limit in France for most drivers. After 24 hours without sleep, the impairment corresponds to about 1 g/L. These equivalences were established by Australian researchers Drew Dawson and Kathryn Reid, in a study published in Nature in 1997. In other words, a sleep-deprived driver can have reaction abilities close to those of a driver over the legal alcohol limit, without being aware of it and without any roadside check able to detect it.

Here's a concrete example: someone who gets up at 6am, works all day, then hits the road at 11pm to reach their vacation destination has been awake for 17 hours by the time they leave. They are already in that risk zone.

Similarly, the length of the previous night's sleep matters a great deal. According to an AAA Foundation study (2016), accident risk rises as soon as someone sleeps less than 7 hours. It is roughly quadrupled after only 4 to 5 hours of sleep, and more than tenfold below 4 hours.

A particular risk with untreated sleep apnea

People with untreated sleep apnea have a road accident risk roughly twice as high as the general population, according to several meta-analyses. This risk stems from the combination of unrefreshing nighttime sleep and daytime sleepiness that is often underestimated by the person themselves. Hence the value of combining screening tools (Epworth for sleepiness, STOP-BANG for sleep apnea) rather than relying on how you feel alone.

The good news is that this excess risk drops markedly once apnea is effectively treated, often within a few weeks. Regulations account for this: in France, a March 28, 2022 order lists the medical conditions incompatible with driving, and excessive sleepiness, whether linked to sleep apnea or another cause, is among them. An affected driver must stop driving until their treatment has proven effective. For truck and public transport licenses, a medical evaluation is required before resuming. Once treatment is properly followed, the vast majority of people resume driving normally.

Warning signs not to ignore while driving

Certain signals should raise immediate concern while driving: repeated yawning, heavy eyelids, difficulty staying in your lane, wandering thoughts, or a few seconds of “blanking out.” When these signs appear, the only effective response is to stop as soon as possible. Opening a window or turning up the radio does nothing to offset the actual drop in alertness.

Other signs are more subtle: not remembering the last few miles driven, missing an exit, drifting onto the rumble strips at the edge of the road, or struggling to maintain a steady speed. At 80 mph, a 4-second microsleep corresponds to more than 450 feet traveled with no control of the vehicle.

Once stopped, the most effective measure is a short 15- to 20-minute nap. A longer one risks tipping into deep sleep, from which you wake up groggy. Studies show that a coffee taken right before this nap boosts its effect: caffeine takes about twenty minutes to kick in, roughly when you wake up. This is only a temporary fix, though. If sleepiness returns quickly, you should plan on actually sleeping before getting back on the road. France's road safety agency also recommends a break every two hours.

Since July 2024, all new cars sold in the European Union must be equipped with a drowsiness-warning system, which notably analyzes steering wheel movements. These systems are useful, but they detect drowsiness once it has already set in. They are no substitute for a good night's sleep before leaving.

Getting an objective measure of your own sleepiness

Many drivers underestimate their own level of sleepiness, especially when it builds up gradually over weeks or months. The Epworth Sleepiness Scale makes it possible to move past this purely subjective impression and, in a few minutes, determine whether the level of sleepiness felt day-to-day goes beyond normal thresholds. It's a useful first step before considering a dedicated medical consultation if needed.

One of the scale's eight situations, in fact, directly concerns driving: the chance of dozing off in a car stopped for a few minutes at a red light or in traffic. If you answer anything other than “never” to that question, that alone is worth mentioning to your doctor, regardless of your total score.