Driving tests

Fatigue and sleepiness: how it affects your driving

There’s only so long you can fight the urge to sleep before your eyes will close and you’ll run off the road, probably with serious consequences. Accidents where drivers fall asleep tend to be at high speed because the driver doesn’t brake. Before that happens, though, there are numerous signs which tell you to stop and sleep.

Tiredness due to lack of sleep causes:

  • Drivers to drift within their lane, moving from one side to another, often being warned about crossing the line by the rumble strips and cat’s eyes.
  • Slow reaction times when hazards develop; things like not spotting that the car in front has braked and then having to brake heavily to avoid a rear-end crash
  • Inability to maintain a consistent speed, i.e. speeding up and slowing down even when there’s no change in speed limit and no traffic ahead
  • Missing information on the road such as signs, turns, motorway exits, etc

Driving tired is like driving drunk!

If you have been awake for 19 hours, it’s similar to being at the legal drink driving limit of 0.05%. Stretch this out to 24 hours and it’s like having a blood alcohol content (BAC) of 0.1%. This is why driving can be so risky for some professions such as junior doctors.

Fatigue and vehicle accidents

Multiple studies have been conducted on sleepy driving. It’s estimated to be a contributing factor in somewhere between 15-25% of all fatal vehicle accidents. Drivers under 25 years old are over-represented in these statistics because they are less resilient to having less sleep.

How does sleepiness start?

Sleep has three components:

  1. Circadian: the biological rhythm that makes us sleepy twice a day, once between 1-6am and again between 2-4pm (siesta time). These are risky times for driving
  2. Homeostatic: the internal drive to go to sleep. When you wake up, this is at its minimum and it gradually increases throughout the day.
  3. Arousal: external and internal stimuli that change how sleepy you are. These include movement (changing your driving position, for example), chemical stimulation such as caffeine, and mental stimulation such as engaging in a conversation.

Arousal effects are short-lived; they can only influence your sleepiness for a short period of time before the other two components take over. For example, opening the window of your car while driving will give you a few minutes of extra stimulation to ward off sleep, but its effect disappears quickly. Turning on the radio might give you a few more minutes. An energy drink and some dark chocolate might give you a quarter of an hour. These are things you should do as an absolute last resort if you know you’re a few minutes away from where you are going, not to indefinitely delay sleep.

Sleep debt

Regularly delaying or shortening your sleep leads to a sleep debt and it’s difficult to catch up. It can be caused by:

  • A disruption to your circadian rhythm, for example by jet lag or shift work
  • Sleep fragmentation: you have a new baby that’s not sleeping well, a storm keeps you awake, or your partner snores
  • Insomnia: you don’t get to sleep at all either because of a medical or psychological condition
  • A partial disruption to your sleep: you went to bed late after going to a party, or you were disturbed a few times during the night
  • Chronic reduction in sleep time: you deliberately get less sleep than you require due to your beliefs or habits

The average person needs 7-8 hours’ sleep; some people can get away with less, but most people can’t. Also, a sleep debt affects people differently. A one-hour reduction in sleep for one person might be the equivalent of three hours’ reduction for another person, depending on how sensitive they are to loss of sleep.

How do you recognise when you’re sleepy when driving?

Different people have different symptoms. Most people have something they do immediately before going to sleep, too, e.g. rubbing your head. However, usually it’s a combination of symptoms that will alert you to the fact that you are not just a little sleepy, but you are way too sleepy:

  • Yawning (usually one of the first symptoms)
  • Lack of concentration (your mind wanders and you realise that you’ve just driven the last 5km and don’t remember a thing about it)
  • Changing your position frequently (unless you are genuinely uncomfortable)
  • Your head starts nodding
  • You have trouble keeping your eyes open
  • Rubbing your eyes and finding it difficult to focus
  • Feeling irritable
  • Difficulty staying in your lane, drifting from side to side
driver training

Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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