Professor Jim Horne

Sleep Neuroscientist − BSc, MSc, PhD, DSc, FSB, FBPsS, CPsych, CBiol





Loughborough Sleep Research Centre

Awake Ltd

Journal of Sleep Research

Jim Horne at

Jim Horne at Google Scholar




Falling asleep at the wheel

The Loughborough Sleep Research Centre has undertaken a considerable amount of research in this area, mostly funded by the UK Department for Transport, and has led to the erection of the motorway signs, ‘Tiredness Kills, Take a Break’, changes to the ‘Highway Code’, and influenced UK Government policy on driver sleepiness.





Throughout Europe, falling asleep whilst driving accounts for around 20% serious collisions under monotonous driving conditions – this incidence rises to around half of the collisions on UK motorways early morning, during the daily trough of our body clock, between 2 and 6 am.   Sleep-related collisions are worse owing to the higher impact speed, and thus are more likely to result in death and serious injury. Many of these collisions are work-related (e.g. trucks, goods vehicle and company car drivers). Around half are caused by healthy men under 30 years old especially when driving in the small hours of the morning, compounded by driving well beyond usual bed-time. These younger drivers not only take more risks in driving sleepy, believing that that they are ‘good drivers’, but don’t realise that sleep loss is more profound in its effects at their age.  Drivers on night-shift work, and driving home after night-shifts are also particularly vulnerable especially during the first night of a night shift.  Older drivers are also more susceptible mid-afternoon during another dip in the body clock, which becomes more evident beyond about 60 years of age.


Under monotonous driving conditions, and at these vulnerable times of the day, it is quite possible for a sleepy driver to fall asleep at the wheel, well within 20 minutes of commencing a drive.


Full adaptation to night-work usually takes at least 10 days continuous nights (with sleep during the day).  One or two interim days off, with sleep reverting back to the night, will negate most, if not all, of any adaptation that may have taken place


Pathological causes of sleepiness largely centre on obstructive sleep apnoea (OSA – also see ‘Very heavy snoring – obstructive sleep apnoea’ AND ‘Overweight truckers and the dangers of undiagnosed sleep apnoea’). Although insomnia is the commonest sleep disorder, there is little or no evidence that non-medicated insomniacs have any greater risk of falling asleep at the wheel – they are more likely to be over-aroused (‘hyperaroused’) rather than be sleepy.  



Falling asleep at the wheel culminates in a series ‘microsleeps’, typified by slow closure of the eyelids followed by slow opening (also called ‘eye-rolling’), usually lasting about 5-7 seconds in total, but this can be for much longer as microsleeps progress (see below).  Needless to say the eyes are unseeing, the brain is in light sleep and steering movements cease and the vehicle may drift (see above).  If the driver wakes up in time there will be momentary confusion and sudden, violent braking and/or steering correction. The driver may become more alert for a while, often only for a minute or so, when another microsleep appears.  The process repeats itself, with microsleeps becoming longer and, maybe, a collision.  Sometimes, in sleepy drivers determined to stay awake, microsleeps appear (as before) without eye closure, but with a blank, trance-like staring through the windscreen.  Again, the eyes are unseeing. ‘Head nodding’ does not always occur during microsleeps as the inclination of the driver’s seat and the driver’s posture may counteract this tendency.


‘Highway hypnosis’ does not exist.  One cannot be hypnotised by the roadway – it is sleepiness. ‘Driving without awareness’ – unable to remember driving for the last 10 minutes or so, is not a sign of falling asleep, merely that the driver was probably on a familiar route with the mind on other matters; still competent to drive.  This happens to us when watching a dull TV programme or reading a newspaper, to the extent that nothing may be remembered, but we are still alert to any new stimulus.


Why is a driver is unlikely to have any recollection of having fallen asleep at the wheel ?

In order to be ‘aware’ that one has been asleep, we have to be asleep for at least a minute or so. Waking someone up from an unscheduled sleep of less than this period of time usually results in the sleeper’s genuine disbelief of having been asleep. Given that most drivers who fall asleep at the wheel cannot drive for such a length of time without spontaneously awaking or having a collision, it is not surprising that they have poor or no recollection of having fallen asleep.  Also, because they were falling asleep/actually asleep at the time of the collision, they often have little recollection of events immediately prior to it.


HOWEVER : With the possible exception of certain clinical conditions (e.g. narcolepsy), sleep does not occur spontaneously from an alert state.  There is a feeling of (often profound) sleepiness beforehand.  It is not possible to be alert one minute and asleep the next.  Sleep is portended by a feeling of increasing sleepiness.  However, despite being very sleepy, drivers (especially young male adult drivers) will often deny the likelihood of actually falling asleep and believe that they can successfully fight sleep.  From our research we have shown unequivocally that drivers who fall asleep in the small hours of the morning declare beforehand that they feel sleepy.  They reach a point of ‘fighting sleep’ by performing acts such as opening the window (for fresh air), turning up the radio, stretching etc, which by the very nature of these acts must demonstrate to the driver that they are sleepy.  Sleep is a dangerous state, and all living organisms are provided with behaviours necessary to ensure that they do not fall asleep spontaneously, and have forewarning to allow them to reach a place of relative safety.


Sleepiness itself clouds ones memory, and it is common for sleepy drivers to have little subsequent recollection of the sleepiness, let alone having then fallen asleep at the wheel.  We have shown that sleepy drivers do know at the time that they are sleepy.  This phenomenon applies to us all, as few of us can remember how sleepy we were before bed-time last night and, particularly, when this became noticeable.  The same applies to hunger and thirst – we can seldom remember either in any detail a few hours after a meal or drink, even though it was clear at the time that we were hungry or thirsty.  The reason is that the human brain does not have the capacity to remember such pointless information after the consummatory acts of sleeping, eating and drinking.


Sleepiness can also impair a driver’s perception of the risk they are undergoing, creating a feeling of optimism about driving capability. When a driver feels the need to open the window, turn up the radio etc, then this is the time to stop driving and take a break.


Take a break – what then?

Simply ‘stretching one’s legs’ or just resorting to other forms of exercise during this break is fairly useless.  Instead, during this break, have a couple cups of coffee (= about 150mg caffeine), or another caffeinated drink having similar levels of caffeine.  Then, and without delay, find a quiet place (ideally back in your vehicle) for a short (15 min) nap, before the caffeine kicks in (this takes at least 20 minutes), followed by a brief walk to freshen up.  This method has been tried and tested by us, confirmed by others, and is now included in the UK ‘Highway Code’


Alcohol worsens sleepiness

Alcohol is not the stimulant that many people seem to think, but has a soporific effect that worsens any sleepiness that happens to be present.  It’s why even a modest drink at lunch-time, a pint of beer, a medium glass of wine or a double shot of spirit, all containing about the same two units of alcohol, and well within the legal driving limit, can lead to an irresistible sleepiness in the afternoon, whereas the same amount in the early evening has little or no such effect, even though changes to blood alcohol levels (BACs) are identical at both times of the day. In fact, by the time this afternoon sleepiness hits you, a couple of hours after that lunchtime drink, there is virtually no alcohol left in the bloodstream.  Even very low BACs as low as 10mg%, well below the legal limit of 80mg per 100 ml of blood (80mg% – equivalent to a breath alcohol level of 35 microgrammes per 100 ml of expired air – 35μ%), can exacerbate any sleepiness and heighten the risk of falling asleep, not just during the afternoon ‘dip’, but worse still during the early morning trough  We have shown that alcohol can continue to worsen sleepiness-related driving impairment at these vulnerable times for the body clock, even when BACs have fallen virtually to zero. Thus, BACs are a poor guide to alcohol-related driver impairment. Lowering the present legal BAC limit to 50mg% in line with other European countries, would help.


Road lighting

We have also investigated whether overhead road lighting reduces early morning sleep related collisions, and find only marginal benefits, particularly as drivers drive faster at this time and, hence, any collision is worse.  Unfortunately, we also find that in the early morning drivers are at their daily least likely to stop and take a break at roadside service/refreshments stops during this