Daytime sleepiness and driving

Feeling tired at the end of a long day is normal. Nodding off without meaning to is something different: your eyes close in front of the television or on the train, and you come round later without ever having decided to sleep. If your sleep plan has picked this up, it's the first thing we deal with, and the reason is mostly about driving.

Two forces pulling against each other

Two systems decide how awake you feel at any moment, an idea sleep research has worked with since the early 1980s (Borbély et al., 2016). The first is the drive to sleep, which sleep scientists call sleep pressure. It builds the longer you're awake and is cleared by sleeping. The second is the body clock, a small cluster of cells in the brain called the suprachiasmatic nucleus (the body's master clock), which raises alertness through the biological day and offsets the pressure as it builds.

Nodding off in the daytime is the drive to sleep winning over the clock's alerting signal. That can happen because the pressure is too high, because sleep hasn't been clearing it properly, or because the clock's signal is at a low point. That signal dips twice in the 24 hours, most deeply in the small hours and again in the mid-afternoon, which is when the road accidents described below cluster.

Where daytime sleepiness comes from

The most straightforward cause is not getting enough sleep. In a laboratory study of 48 healthy adults aged 21 to 38, Van Dongen and colleagues (2003) held some of them to 4, 6 or 8 hours in bed a night for two weeks. On six hours or less, attention and thinking got steadily worse, and by the end the deficits matched up to two nights with no sleep at all. Their own ratings of sleepiness rose at first and then barely changed, so they were largely unaware of how impaired they'd become. Feeling that you're coping can't be taken as proof that you are.

Broken sleep has a similar effect, because time awake in the night doesn't clear the pressure. Sometimes the cause is a sleep condition. In obstructive sleep apnoea, for example, breathing stops and starts repeatedly during sleep. The NHS lists loud snoring, gasping or choking noises in the night and feeling very tired during the day among its signs, and advises seeing a GP if you have them.

Why it matters most behind the wheel

Horne and Reyner (1995) looked at 679 road accidents in south-west England and the Midlands where the most likely cause was the driver falling asleep. These made up 16% of the accidents police attended on major roads in the south-west and over 20% on Midlands motorways. They clustered around three times of day, about 02:00, 06:00 and 16:00, which match the low points in the body clock's alerting signal. The study counted accidents and couldn't compare them with how many people were on the roads at each hour, but the timing pattern agrees closely with laboratory work on daily rhythms of sleepiness.

The government's guidance on excessive sleepiness and driving describes it as having difficulty concentrating and finding yourself falling asleep. It says you must not drive until you're free from excessive sleepiness, or until your symptoms are under control and you're following any treatment you need. Telling the DVLA is a separate matter. It applies to certain diagnosed conditions, such as moderate or severe obstructive sleep apnoea with excessive sleepiness, or to another sleep condition that has caused excessive sleepiness for three months or more, and your GP can tell you whether it applies to you.

The Highway Code (rule 91) also advises against starting a journey tired, recommends regular breaks on long journeys, and says that if you feel sleepy you should stop somewhere safe, which doesn't include the hard shoulder of a motorway.

Why your plan puts driving first

Everything else in your plan is about sleeping better; this part is about safety, for you and for other people on the road. If you've told us you nod off without meaning to, the plan will ask you not to drive for now and to see your GP, and being seen isn't the same as being told it's safe to drive again. Your GP is the person who can work out the cause, whether that's too little sleep, broken sleep or a condition that needs treating.

Alongside that, your diary shows how long you give yourself in bed and how much of it is sleep, and small changes to timing and time in bed are how we try to bring the pressure down. If the nodding off continues once your nights look adequate in the diary, that points even more firmly back to your GP.