Too many of us have been there: pushing through a long drive, trekking just a few more miles as our eyelids gradually become weighted by the need for sleep. More often than not, we slip away from the impending danger — we escape an accident, injury, or even death. The unlucky drowsy drivers do not. What contributes to the hazard of drowsy driving? How can it safely be avoided? Learn about the link between sleep deprivation, sleep disorders, and drowsy driving, and discover ways to keep you and your loved ones safe on your next trip.
What Causes Drowsy Driving?
It isn’t a mystery why people feel drowsy while driving: too little sleep, or sleep of poor quality, plays a central role. But the underlying cause of the excessive sleepiness may vary. In addition, the timing of the drive itself may contribute to the risk.
Wakefulness and drowsiness depend on two separate, overlapping processes. The first is something called the homeostatic sleep drive. Simply put, this is the fact that the longer you stay awake, the sleepier you get, and the more likely you are to fall asleep. This is due to the build up of a chemical or neurotransmitter within the brain called adenosine. The presence of adenosine contributes to drowsiness, and during sleep, it's cleared away.
The other process that contributes to our feeling sleepy is the circadian rhythm. The body follows a natural pattern that is approximately 24 hours in length, and body temperature, energy use, hormone levels, and sleep follow regular patterns. The circadian rhythm provides an alerting signal that counteracts the increasing levels of adenosine throughout the day. When the circadian signal drops off at the start of our sleep period, we fall into a deep sleep. And if we're awake during the time that our body expects us to be asleep, we're more likely to doze off.
Therefore, a poorly-timed drive may put us at risk for falling asleep at the wheel. If it's at the end of a long day, or if it occurs overnight, we'll be at greatest risk. As Dr. William C. Dement, one of the founders of sleep medicine and a professor at Stanford University, warns his students as part of his Sleep and Dreams course, "Drowsiness is a red alert." It's your body’s warning that you're about to fall asleep, and it's your chance to pull off the road and save your life. Why is the sensation in the context of driving so dangerous?
The primary danger with drowsiness is that you're at extreme risk of falling asleep. In particular, you may suffer a brief episode of microsleep, which may cause your head to bob before you jerk awake, and may send you across the rumble strips at the edge of the road. In the worst-case scenario, at high speeds, you may be sent off the road entirely.
In addition, drowsiness may subject you to automatic behaviors. The most classic example is sometimes called "highway hypnosis." In a sleepy state, drivers may be able to conduct their vehicle over long distances without focusing on the external environment. Such drivers may suddenly find themselves at their destination, failing to recollect long stretches of the journey. Some may pass their intended exit off the highway; this seems to represent a form of dissociation, in which the mind focuses elsewhere while minimal attention is maintained to perform a task. In periods of sleepiness, this risk is likely increased.
The easiest way to become sleepy is to fail to meet your individual sleep needs. Although it varies, most people need 7 to 8 hours of sleep to feel rested. But if you happen to need 10 hours, and only get 8, you'll be sleep-deprived. It is possible to determine your sleep needs. One way to assess for excessive daytime sleepiness is to complete the Epworth sleepiness scale. It can also be tested with a multiple sleep latency test (MSLT). In addition, the maintenance of wakefulness test (MWT) is often used in truck drivers and airline pilots to ensure safety.
Sleep disorders may also affect your quality of sleep, leading to disruptions that undermine its restorative powers. In particular, conditions like sleep apnea and narcolepsy frequently lead to excessive sleepiness. Interestingly, individuals with insomnia are rarely sleepy during the day: they're unable to take naps if given the opportunity, for instance. Instead, they frequently complain of fatigue and are "tired but wired." Untreated sleep disorders may be a major contributor to drowsiness during driving, but the most common by far has to be simple sleep deprivation.
The Role of Sleep Deprivation in Traffic Accidents
Research over the past 20 years has verified the critical effects of sleep deprivation on the ability to drive safely. Quite simply, driving drowsy will increase your risk of having a traffic accident. It can certainly be linked to falling asleep, but a lack of attention and concentration also play a role.
Research has carefully assessed the risk of driving in a sleep-deprived state with the use of driving simulators. These studies demonstrate a profound loss of driving ability and safety. In some studies, sleep deprivation may lead to impairment that is similar to being legally drunk. If you wouldn’t drive drunk, why would you risk driving sleepy?
It's been demonstrated that sleeping fewer than 7 hours per night on average is particularly risky. Those who have poor sleep quality and excessive sleepiness are also more likely to have traffic accidents. Finally, as would be expected, driving at night also results in more frequent accidents among those who are sleep deprived. How can drowsy driving, and the associated risk of car accidents, be avoided?
Ways to Avoid Drowsy Driving
In order to avoid the risks associated with drowsy driving, there are interventions that you can make to ensure your safety. The most obvious is also the easiest: get enough sleep! If you're planning to take a long trip, be certain that you're well rested before you set off. And even if you're merely engaging in your daily commute, you should only get behind the wheel if you're not sleepy. If you're feeling sleepy, you can take a brief nap before starting to drive.
In light of the information detailed above, you will be less drowsy if you drive in the morning (at least 1 hour after waking up to avoid sleep inertia). Long overnight drives are especially dangerous, as this is when your body expects you to be asleep — and will encourage you to do just that. In addition, uninterrupted trips on straight, unvarying terrain may be hazardous. With less mental engagement, it's more likely for you to tune out and become less attentive. If you're faced with a long, boring drive, you'll do best to take frequent breaks to interrupt the tedium.
It may also be helpful to have company on your journey. Someone else in the car can be stimulating for conversation while you drive. She may also recognize when you're getting drowsy, or you may express that you are, and she may be able to take over driving for you.
Caffeine can be a helpful aid to improve attention: it's able to block the effects of the increasing levels of adenosine. It has its limits, however, and cannot be a substitute for adequate sleep. Other stimulants, including medications such as Provigil, Nuvigil, and Ritalin, may also be useful in the proper context. In particular, they may be part of the necessary treatment for sleep disorders, which should be optimally managed to avoid adverse effects on driving.
A few common activities that are meant to help counteract drowsiness while driving are simply not effective. Rolling down the windows or turning up the radio will unfortunately not keep you awake: when tested in driving simulators, people who attempted these maneuvers still fell asleep at the wheel. So don’t rely on these two interventions to keep you safe.
The Consequences of Being Too Drowsy to Drive
What are the consequences of being too drowsy to drive? The most serious, and potentially fatal, is that you are at a higher risk of having a car accident. This may not be enough to deter you. You may be drowsy every day, but you may never have had a serious accident. Unfortunately, the time that you do finally crash your car may be too late.
Depending on your state of residence, there may also be legal consequences. Not only may you face citations for traffic violations, you may lose your driver’s license. In many states, physicians may be required as mandatory reporters to notify the local government of your risk of excessive drowsiness behind the wheel, which may result in license restrictions until your condition is improved.
If you remember nothing else, recall this simple statement when driving, and take it to heart: "Drowsiness is a red alert."
Kryger, MH et al. "Principles and Practice of Sleep Medicine." ExpertConsult, 5th edition, 2011.