Driver Knowledge Tests

How does being tired affect your driving?

We have a sleep/wake cycle that affects how tired we are. We can fight this cycle, but only for so long: eventually, our body’s requirement for sleep will be too strong and we will fall asleep. If this happens while you are driving then the results can be quite disastrous.

How does lack of sleep affect your driving?

Lack of sleep causes a driver to:

  • Vary their driving speed, gradually slowing down then realising, so they speed back up again, then repeating that cycle.
  • Vary their lane position, finding it hard to follow a straight line within the lane, often being warned about crossing the lane lines by running over lane markings or cats’ eyes.
  • Have a slow reaction time to hazards, often having to brake late because they didn’t notice a sharp corner or a vehicle ahead that had slowed down.
  • Miss hazards due to a reduction in the ability to perceive what a hazard is.

When you’ve been awake 17-19 hours, it has the same effect on your driving as having a blood alcohol content (BAC) of around 0.05%, which is the same as the legal limit. When you’ve been awake for more than 24 hours it’s the same as having a BAC of around 0.1% or twice the legal limit. Bear in mind that losing sleep over a long period of time can cause a sleep debt which can have the same effect.

What’s a sleep debt?

If you consistently get less than the amount of sleep you need, you’ll build up a sleep debt and this can be caused by:

  • A partial reduction in sleep time, e.g. you were disturbed during the night, or you went to bed late but had to get up early
  • A chronic reduction in sleep time, e.g. you have to shorten your sleep hours on a repeated basis
  • A total elimination of sleep, e.g. you don’t get any sleep during the night due to insomnia, or you were kept awake on a long flight
  • Sleep fragmentation, e.g. you have a new baby, your partner snores, etc
  • Circadian disruption, e.g. you have sleep apnoea, you do shift work, you have jetlag, etc

The majority of people need 7-8 hours’ sleep, but a reduction in sleep can affect different people in different ways. For example, a 1-hour reduction of sleep for one person might be the equivalent of a 2-hour sleep reduction for another person.

What causes sleep?

There are three components to sleep.

  1. The homeostatic component is your internal drive to go to sleep. From the moment you wake up, the homeostatic component builds until you go to sleep again.
  2. The circadian component is the biological rhythm which, twice a day (between about 1-6am and 2-4pm) makes your body tired and more prone to sleep. It’s best not to drive during these times, if possible.
  3. The arousal component consists of stimulation that reduces your sleepiness.


Things that influence sleep arousal are:

  1. Movement, e.g. changing driving position
  2. Mental stimulation, e.g. listening to a podcast
  3. Chemical stimulation, e.g. caffeine, cocaine (note: you shouldn’t be using cocaine and driving)

Arousal effects don’t last very long and do nothing to change the underlying requirement to sleep. The only way is to actually get some sleep.

If you are sleepy and you need to reduce your sleepiness, a nap of 20 minutes is the best option. Much more than this and you’ll feel groggy when you wake up. The least effective measures are opening the window, singing, listening to music, energy drinks and talking to a passenger.

How many crashes are caused by sleepiness?

According to a study by the Centre for Accident Research & Road Safety at the Queensland University of Technology, 57% of drivers drive when sleepy, 73% of drivers continue to drive once aware of their increasing levels of sleepiness, and drivers under 25 are over-represented in sleep-related crashes.

A study by AAA Foundation in America found that 21% of all fatal vehicle accidents involved a fatigued driver.

There’s more risk of being in a sleep-related crash if you are driving between 1-6am and 2-4pm, you do shift work, you have unrealistic work schedules and you’re in poor health. Fatigue is a particular problem for long-distance truck drivers who drive at night to avoid traffic.

When you are fighting sleep your body starts to take microsleeps where it shuts down for a few seconds at a time. Ignore these and your body will enter into proper sleep.

Signs of sleepiness

If you notice any of these then you should start to think about pulling over and having a short nap.

  • Increased blinking
  • Lack of concentration
  • Increased frequency of changing your posture (obviously excluding other causes like that you have cramp)
  • Your head is nodding
  • Yawning
  • Rubbing your eyes
  • Drifting within your lane
  • Difficulty keeping your eyes in focus
  • Irritability
  • Difficulty maintaining the same speed

Find out more causes and symptoms of fatigue.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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