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Fatigue is thought to be the cause of a large number of crashes involving a car striking a tree or other rigid object, and severe head-on collisions. Researchers believe fatigue is involved in between 10% and 25% of all crashes and it accounts for around 20% of all serious accidents on motorways.
 
There's a lot more to driver fatigue than simply nodding off at the wheel. Falling asleep is of course a form of fatigue, but it's the most extreme form, occurring only after a number of other - easily recognisable - symptoms have been ignored. Your driving can be affected by fatigue long before you are so tired that you are in serious danger of falling asleep at the wheel. 

The symptoms

Fatigue affects your driving in many ways. The most obvious early symptom is a reduction in your ability to concentrate and focus on what's going on around you.

It will take you longer to get to grips with information which you would interpret immediately when fully alert. For example, you could find yourself simply staring at a road sign - or worse - a hazard ahead, without being able to interpret the message it's offering and therefore being unable to react promptly.

You may also find - especially on a long, dull motorway journey - that you may be drifting out of your lane. You may think you're going at a constant speed, but it's common among fatigued drivers to change speed more frequently and for no good reason. You may also find yourself fidgeting in your seat in an attempt to wake up a bit.

Deadly combinations

Driver fatigue becomes even more of a risk when it combines with other factors, such as alcohol consumption and speeding. Drink-driving is particularly dangerous in combination with fatigue, because alcohol can affect a driver's alertness long before he or she reaches the legal drink-drive limit. Any amount of alcohol can combine with fatigue to affect your driving.

Driving at higher speeds when affected by fatigue also increases risk, as a fatigued driver with slower reactions will need more time to react to events unfolding ahead of him.

What causes fatigue?

One of the most common causes of fatigue is, unremarkably, lack of sleep. An average daily sleep requirement is between seven and eight hours - but yours may be different. If you miss your full night's sleep, you'll feel tired the next day. Researchers have found that as little as two hours sleep loss on one occasion can affect your reaction time, mental functioning, memory, mood and alertness. Allow your sleep loss to go unchecked across several nights and you build a significant sleep debt. If you let this become too large, then your brain will eventually go to sleep involuntarily (micro-sleep), regardless of where you are and what you are doing.

Micro-sleeps generally only last a few seconds, but can obviously be very dangerous if they occur while you're driving. For example, during a two-second micro-sleep at a speed of 60 mph, a car will have travelled around 56 metres without you in control.

Don't worry that you may be caught by sleep with no warning. Professor Jim Horne from Loughborough University explains clearly: "Sleep doesn't come spontaneously from nowhere. You can't be driving along alert one minute and falling asleep the next. There's always adequate time to realise how sleepy you are."

People who are most likely to be affected by fatigue

Although we are all likely to experience some level of fatigue, it is more prevalent among the following groups:
 
* Young people, with lifestyles that involve 'burning the candle at both ends', going to parties, staying up late, taking risks and being on the road at night.

* Shift workers, whose disrupted sleep patterns can easily lead to fatigue. Night shift workers have the greatest risk of sleep disruption.

* People suffering from sleep disorders can of course have the quality and quantity of their sleep affected. The most common disorder is sleep apnoea, where the sleeper's throat relaxes so deeply that he or she actually stops breathing. The sleeper gasps, wakes up enough to start breathing normally, and then goes back to sleep without being aware of any problem. This occurs as often as 600 times a night, leaving the apnoea sufferer thinking he has had a full night's sleep, yet inexplicably tired throughout the day.

Wake up to the dangers
 
* Make sure you get plenty of sleep before a long journey. Plan to drive during times of the day when you're normally awake, don't push yourself to complete a long journey all in one go. Schedule a night stop somewhere - it makes sense to take a break after lunch and to be asleep between midnight and 6 am.

* If you're on a long journey, the Highway Code advises a 15-minute break every two hours as a minimum. Take more frequent breaks if you need to. Get out of the car, have some fresh air, stretch or exercise a little. If you're tired, then take a nap if it's safe.

* Don't delay taking a nap. A short sleep of 15 to 30 minutes can be helpful, but if you sleep more than 40 minutes you will find it hard to wake up and be alert - at least for 15 minutes after waking.

* Avoid heavy meals on journeys, especially at lunchtime, as these can exacerbate sleepiness in the afternoon.

* Drinks with a caffeine content can help you stay alert, but remember, their effects are not instant. Research shows that taking a caffeinated drink, followed by a 20-minute nap, appears to be the best short-term measure for combating fatigue.

Other tips for staying alert:
 
* Open the windows from time to time, to get some fresh air into the vehicle.

* If possible, share the driving.

* Make sure you're not taking any medications that could make you drowsy.

Warning:
The advice offered here for dealing with fatigue constitutes little more than emergency measures to ensure you can reach a rest stop in safety. And remember, the symptoms will simply return if you don't have the opportunity for proper, quality rest. There is no substitute for being well rested before a journey.

Our additional information sources: RoSPA (www.rospa.com), Loughborough University Sleep Research Centre (www.lboro.ac.uk), Land Transport New Zealand (www.ltsa.gov.nz)

David Williams was awarded the MBE in 1998 for services to Road Safety. He is Chief Executive of GEM Motoring Assist, who sell the best breakdown cover covering both car and caravan recovery.