The Effects of Sleep Deprivation on Driving: How Bad is It?

Just about everyone knows that driving while tired is not a good idea.

But just how bad is it?

I’ve summarized several studies in this short post that look at this very topic.

Note that pretty much all research is done on short-term sleep deprivation (i.e. having subjects stay up all night) instead of long-term chronic sleep deprivation, which may have different effects.

Additionally, we’ll compare the effects of sleep deprivation vs alcohol on driving.

Sleep Deprivation Reduces Reaction Time

It turns out that sleep deprivation doesn’t affect all aspects of driving equally.

One study had 19 professional drivers go through a period of 27 hours of total sleep deprivation before testing their driving abilities (1). They found that:

Simulated driving performance and neurocognitive measures of vigilance and reaction time were impaired after sleep deprivation, whereas tasks examining processing speed and executive functioning were not significantly affected by sleep loss

In other words, the parts of unconscious driving like reaction time were significantly impaired.

On the other hand, the situations where the drivers needed to consciously think to assess a situation were not affected. Now obviously you still need to react to those thoughts, so there’s still an overall impact.

SummaryA single night of total sleep deprivation significantly impaired the reaction times of professional driving subjects in a simulated environment.

Sleep Deprivation Reduces Reaction Time at Certain Times of Day

There are times after not sleeping for a night where you actually feel full of energy.

One research team wanted to see if there was actually anything behind this feeling.

They had subjects complete driving tests using a motorcycle after not sleeping for a night (2).

The most important finding was that sleep deprivation affected driving speed, especially later on in the day:

Emergency braking performance is affected at both speeds by time of day, with poorer performance in the morning, and also by sleep deprivation

To make that a bit clearer, the results showed that all drivers stopped more slowly in the morning, sleep impaired or not.

In addition, drivers who were sleep deprived took longer to stop, especially in the morning.

The graphs below show the stopping distance at both 20 kph and 40 kph:

The difference isn’t huge, but even a 10% longer stopping time can be the difference between an accident or not.

SummarySleep deprivation affects several areas of driving ability, including increasing the distance needed to stop. This is most apparent at lower speeds where reaction time is most important.

The Effects of Sleep Deprivation on Longer Drives

Another interesting area to look at is if sleep deprivation affects both short trips and long trips equally.

One research team looked into this by testing 20 male subject drivers (3). Before the sleep deprived test, they were only allowed 4 hours of sleep during the night between 3 and 7 AM.

While they looked at several metrics, I found the number of right edge-line crossings (i.e. crossing into another lane) as the most interesting results.

The chart below really illustrates that each subsequent 10 minute driving period resulted in more driving errors:

While this applies even when not sleep deprived, the drivers made significantly more errors when sleep deprived, especially after the first few driving tests.

SummaryThe amount of driving done is a predictor of unforced driving errors like crossing over lane boundaries. Sleep deprivation increases the amount of these errors over trips of all lengths, but especially long trips.

Full Sleep Deprivation vs Partial Sleep Deprivation vs Alcohol

Finally, let’s compare the effects of sleep deprivation with driving under the influence of alcohol. There are a few studies that have looked at this topic.

The first study compared 24 hours of wakefulness to a BrAC of about 22 μg/100mls (about the legal limit in most places) (4).

They found that both driving under the influence of alcohol and sleep deprivation had a significant effect on overall driving quality:

In the chart above, a higher score indicates worse driving.

It turned out that sleep deprivation is arguably worse than sleep deprivation. Drivers under sleep restriction scored worse in 2 key areas:

Compared to alcohol, sleep deprived mean reaction times were slower (2.86 s vs. 2.34 s) and lateral control of the vehicle was reduced (lane tracking adaptive mean deviation: 0.5 vs. 0.3). 

In addition, the researchers tested whether or not coffee was helpful in those who were sleep deprived and found that performance deteriorated even further.

Another study also compared the effects of a night of sleep deprivation to alcohol, but introduced a partial sleep deprivation group (4 hours of sleep) as well (5).

One metric they looked at was the number of lane crossings, shown in the graph below:

Overall, the partially sleep deprived subjects still drove about as safe as usual, but both other groups had much worse performances:

The results revealed that the full sleep deprivation and alcohol group exhibited a safety-critical decline in lane-keeping performance. The partial sleep deprivation group exhibited only noncritical alterations in primary task performance.

One final note of interest is that the sleep deprived groups were aware that their performances were worse than usual, but the alcohol group did not have this subjective awareness.

SummaryLet’s put this in context. Driving under the influence is one of the most selfish and reckless things to do, it endangers the driver and everyone else on the road. While the research we looked at isn’t a complete picture, it appears that driving while fully sleep deprived is possibly just as bad as driving drunk.

References

1. Cognitive components of simulated driving performance: Sleep loss effects and predictors
2. Effects of Time of Day and Sleep Deprivation on Motorcycle-Driving Performance
3. Effect of driving duration and partial sleep deprivation on subsequent alertness and performance of car drivers
4. The impact of sleep deprivation and alcohol on driving: a comparative study
5. Impairment of Driving Performance Caused by Sleep Deprivation or Alcohol: A Comparative Study