Why driving while tired is WORSE than driving drunk.

It is illegal in many places to drive a car under the influence of alcohol, which is often defined as having a blood alcohol level (BAC) greater than .08%. At this BAC, your reaction to the environment is slower, your regulation of attention is poorer, and your impulsivity is higher. Most of us know this, and most of us don't drink and drive. Thank you for that. However, it begs the question of whether there are other factors that can impair our driving the way alcohol can. 

Begrudgingly, I am awake at about 6:30 each morning in order to be at work by 8. On most days, I'm asleep by midnight, but on Friday nights there are a number of fun things to do that can keep me up much later than midnight. For example, this weekend I was at the wedding of a close friend in South Bend, Indiana which required me to take the 11:00pm red eye from San Diego to Chicago, and then drive to South Bend early in the morning to make the ceremony at the Basilica of the Sacred Heart by 11am. Needless to say, I was tired. Despite knowing that I was tired, I was completely confident that my mere 4 hours of intermittent sleep on the flight wasn't going to impair my ability to drive to South Bend. Given the same circumstances, I'm sure many of you would have done the same. However, given that we know how impairing not sleeping can be to our cognitive functioning, it would only make sense for lack of sleep to also impair our ability to drive.

In fact, a study was conducted by Dr. Williamson and colleagues at the University of New South Wales to investigate the differences in cognitive impairment between alcohol consumption and sleep deprivation. 

To conduct this study, they recruited 39 adults, most of whom were between the ages of 30-49, and randomly assigned them to two groups for a study that took 3 days. The first group arrived at the lab and practiced a few cognitive tasks measuring memory, attention, impulsivity, reaction times, and reasoning. Then the participants slept in a local hotel room where they were given the opportunity to get a solid night's sleep. The next morning, the participants returned to the lab for the segment of the study assessing the effect of alcohol on cognitive abilities.  They were then given hourly doses of their cocktail of choice (any spirit and any mixer). Then, every 30 minutes, the participant repeated the cognitive tests, so that there were assessments of each participant's cognitive abilities at BACs of .025%, .05%, .075%, and .10%. At the end of the day, these participants returned to their hotel to get a good night's rest. The next morning, the participants returned to the lab for the segment of the study assessing the effect of sleep deprivation on cognitive abilities. Participants were asked to completed the cognitive tasks, at first every hour and then every 2 hours, during a 28 hour period. 

To account for any sequence effects of drinking before sleep deprivation, the second group of participants completed the sleep deprivation segment first and the alcohol consumption segment second. 

The results of this study will blow you away. Indeed, they found that alcohol resulted in impaired reaction times (15% reduced), hand-eye coordination (10% reduced), a 200% increase in missed signals, and a 50% increase in false alarms responses during different tests. In other words, under the influence of alcohol, you are slower to react to the environment, have worse coordination, miss what you're supposed to react to, and react to cues that you are supposed to ignore. None of this was surprising. However, when you compare these findings to that of sleep deprivation, you find that by the time you have been awake continuously for 17 hours (for me that would be at around 11:30pm, your brain is operating similarly to how it operates at a BAC of .05%. And by the time you have been up for 19 hours, you are thinking like you are at a BAC of .10%. Yikes! 

In fact, in some domains, sleep deprivation is worse. In this study, more than 75% of all participants showed worse performance on the reaction time, spatial memory, hand-eye coordination, and vigilance tasks when they had been awake for 17 straight hours than they did when they had a blood alcohol level of .05%. That's not even the legal limit for driving. 

So, what does this mean? In short, the longer you've been awake, the more your cognitive capacities are impaired. You should take driving while tired as seriously as you do driving under the influence of alcohol. But how feasible is that recommendation? Many of us work hard, and sometimes you are just not keeping track of how long you've been awake. What if you are pulled into a late project that just has to meet a deadline? Should you not drive home when you're finished. This data would say no, it's probably not as safe as you think, but we all know you're still going to do it. 

What I've taken from this is something a bit more manageable. Often times in the evening I reach a breaking point where I no longer need to be at the office, but still have work to do. This data would suggest that driving home earlier, even though working from home is less efficient, and more distracting, is a safer choice to make. More importantly, the lesson to be taken from this study is an increased awareness. Now that you know how much just being awake all day long can impact your driving, hopefully you will use more caution when driving home from a dinner party or a late meeting. 

One limitation of this study was that all but 2 of the participants were male, so we have no idea whether the effects would be different in women, but from what we know about the biology of sleep, it's unlikely. Stay safe out there! 

Williamson, A.M. & Feyer, A-M. (2000). Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication. Journal of Occupational and Environmental Medicine, 57, 649-655.