Who, what, why: What is a micro-sleep?
Almost half of male drivers admit to experiencing micro-sleeps at the wheel. What are they?
It's a "horrifying" statistic, according to road safety charity Brake.
Of 1,000 drivers it interviewed, 45% of men admitted to micro-sleeping while driving, as did 22% of women. But what does this mean?
Micro-sleep is an episode of light sleep lasting five to 10 seconds. The brain goes to sleep involuntarily and it is more likely to happen in a monotonous situation. People wake suddenly, often with a sharp jerk of the head.
"Your eyelids start drooping and you start to lose contact with reality," says Prof Jim Horne, director of Loughborough University's Sleep Research Centre. "You're asleep for a few second, then wake up, often with a jolt."
- Light sleep that lasts from five to 10 seconds during which the brain goes to sleep involuntarily
This sudden head jerk is how people commonly know they've had a micro-sleep as the brain doesn't remember such short naps.
"Sleep has to last beyond a minute or two for your brain to remember it," says Prof Horne, who studied driver tiredness for 10 years. "With micro-sleep, you are just left with a feeling of not knowing if you are coming or going."
It is caused, unsurprisingly, by fatigue. If episodes aren't tackled, they become more frequent and last longer until you eventually fall into a proper sleep.
A head jolt in dangerous or inappropriate circumstances, like driving a car or in a meeting, can sometimes prevent further episodes as the shock and realisation of what has happened sends adrenaline pumping through the body.
Find out more about sleep
Drivers are particularly susceptible as driving can be monotonous. They are more at risk in the afternoon as the body experiences a drop in energy levels, also at night when a person is usually asleep.
Young drivers are also more at risk because young people need more sleep in general, so are more impaired by sleep loss.
It's estimated that almost 20% of accidents on dull, major roads are sleep-related, according to the Department for Transport. Drivers have no excuse, says Prof Horne.
"Sleep doesn't come from out of the blue. 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."
His advice, included in the Highway Code, is to safely park then have a drink containing about 150mg of caffeine. It will not kick in for 20 minutes, so have a 15-minute nap and then freshen up for five minutes before continuing your journey.
Reporting by Denise Winterman