Loughborough University
Leicestershire, UK
LE11 3TU
+44 (0)1509 263171

School of Sport, Exercise and Health Sciences

Loughborough Sleep Research Centre

Publications - abstracts from papers

Driver sleepiness

1) General aspects

Horne J.A. & Reyner L.A. Sleep Related Vehicle Accidents. British Medical Journal, 1995, 310, (6979), 565-567.

Our objective was to assess the incidence, time of day, and driver morbidity associated with vehicle accidents where the most likely cause was the driver falling asleep at the wheel. Design Two surveys were undertaken, in SW England and the Midlands, using police data bases or on-the-spot interviews. A total of 680 sleep- related vehicle accidents (SRVAs) were identified. Results Of all vehicle accidents to which the police were summoned, SRVAs comprised 16% on major roads in SW England, and over 20% on Midland motorways. During the 24h period there were three major SRVA peaks, at around 02:00h, 06:00h, and 16:00h. About half these drivers were men under 30y; few SRVAs involved women. SRVAs are largely dependent on the time of day, and account for a significant proportion of vehicle accidents, especially those on motorways and other monotonous roads. As there are no UK norms on road usage by age and sex for time of day with which to compare these SRVA data, we cannot determine what the hourly exposure vs risk factors are for these sub-groups. Our findings are in close agreement with those from other countries.

Horne J.A. & Reyner L.A. Vehicle accidents related to sleep: a review. Occupational & Environmental Medicine, 1999, 56(5): 289-294.

Falling asleep while driving accounts for a considerable proportion of vehicle accidents under monotonous driving conditions. Many of these accidents are related to work-for example, drivers of lorries, goods vehicles, and company cars. Time of day (circadian) effects are profound, with sleepiness being particularly evident during night shift work, and driving home afterwards. Circadian factors are as important in determining driver sleepiness as is the duration of the drive, but only duration of the drive is built into legislation protecting professional drivers. Older drivers are also vulnerable to sleepiness in the mid-afternoon. Possible pathological causes of driver sleepiness are discussed, but there is little evidence that this factor contributes greatly to the accident statistics. Sleep does not occur spontaneously without warning. Drivers falling asleep are unlikely to recollect having done so, but will be aware of the precursory state of increasing sleepiness; probably reaching a state of fighting off sleep before an accident. Self awareness of sleepiness is a better method for alerting the driver than automatic sleepiness detectors in the vehicle. None of these have proved to be reliable and most have shortcomings. Putative counter measures to sleepiness, adopted during continued driving (cold air, use of car radio) are only effective for a short time. The only safe counter measure to driver sleepiness, particularly when the driver reaches the stage of fighting sleep, is to stop driving, and-for example, take a 30 minute break encompassing a short (<15 minute) nap or coffee (about 150mg caffeine), which are very effective particularly if taken together. Exercise is of little use. Conclusions-More education of employers and employees is needed about planning journeys, the dangers of driving while sleepy, and driving at vulnerable times of the day.

Horne, J.A. & Reyner L.A. Sleep-related vehicle accidents: some guidelines for road safety policies. Transportation Research - Part F, 2001, 4: 63-74.

Sleep-related vehicle accidents (SRVAs) are a common form of highway accident, often wrongly attributed to other causes. SRVAs typically involve running off the road or into the back of another vehicle, with no braking beforehand. Because of a high impact speed these accidents are often serious. SRVAs peak around 02:00-06:00h and 14:00-16:00h, when daily sleepiness is naturally higher. Hence, time of day is a critical factor, as important as the duration of the drive. Most SRVAs are not due to sleep pathology. Many are work-related. Non-sleeping "rest" is no substitute for sleep. Sleep does not occur spontaneously without warning, and is preceded by feelings of increasing sleepiness of which drivers are quite aware. Driving impairment is usually worse than is realised by the sleepy driver. The best countermeasure is sleep, or even a short nap. Even more effective is the combination of a nap with caffeine.

Horne, J.A. & Reyner, L.A. Driver sleepiness and sleep disorders, with particular reference to Parkinson's disease. European Journal of Neurology, 2000, 7 (Suppl 4): 15-20.

Falling asleep while driving accounts for a significant proportion of vehicle accidents under monotonous driving conditions. Time of day (circadian) effects are profound, and are as important in determining driver sleepiness as is the duration of a drive. Older drivers are particularly vulnerable to sleepiness in the mid-afternoon. Only a minority of these accidents seem to be due to pathological causes of sleepiness. Patients liable to increased daytime sleepiness must be told that driving in this state is dangerous and that they have a duty of care toward other road users. Parkinson's disease (PD) often causes a variety of night-time sleep disturbances and excessive daytime sleepiness, although the association between the two may be complex. There is little evidence to show that PD can result in sudden, unforewarned 'sleep attacks'. In healthy people, sleep does not occur spontaneously without warning. Although healthy drivers falling asleep are unlikely to recollect having done so (also claiming 'sleep attacks'), they would have been aware of the precursory state of increasing sleepiness; probably reaching a state of fighting off sleep prior to an accident. Putative countermeasures to sleepiness, adopted during continued driving (e.g. cold air, use of car radio) are only effective for a short while, even in healthy, sleepy individuals. The only safe countermeasure to driver sleepiness, particularly when the driver reaches the stage of fighting sleep, is to cease driving-and, perhaps, take a 30-min break encompassing a short nap (<15 min) and/or coffee (about 150 mg caffeine).

2) Combined with an alcohol intake well under the UK legal limit

Horne J.A., Reyner L.A., & Barrett P.R. Driving impairment due to sleepiness is exacerbated by low alcohol intake. Occupational & Environmental Medicine, 2003, 60: 689-692.

Objectives - To assess whether low blood alcohol levels (BACs), around half the UK legal driving limit, and undetectable by police roadside breathalysers, further impair driving already affected by sleepiness, particularly in young men, who are the most "at risk" group of drivers for having sleep related crashes. Methods -12 healthy young men drove for 2h in the afternoon, in an instrumented car on a simulated motorway. In a repeated measures, counterbalanced design, they were given alcohol or placebo under conditions of normal sleep or prior sleep restriction. Measurements were: driving impairment (lane drifting), subjective sleepiness, and EEG measures of sleepiness. Results - Whereas sleep restriction and alcohol each caused a significant deterioration in all indices, the combined alcohol and sleep restriction further and significantly worsened lane drifting (which typifies sleep-related crashes). This combined effect was also reflected to a significant extent in the EEG, but not with subjective sleepiness. That is, alcohol did not significantly increase subjective sleepiness in combination with sleep loss when compared with sleep loss alone. Conclusions - Modest, and apparently "safe" levels of alcohol intake exacerbate driving impairment due to sleepiness. Our sleepy drivers seemed not to have realized that alcohol had increased their sleepiness to an extent that was clearly reflected by a greater driving impairment and in the EEG.

Barrett P.R., Horne J.A. & Reyner L.A. Early evening low alcohol intake also worsens sleepiness-related driving impairment. Human Psychopharmacology, 2005, 20: 287-290.

Following night-time sleep restriction, afternoon driving performance during the bi-circadian surge in afternoon sleepiness is markedly worsened by blood alcohol concentrations (BACs) well under most national driving limits. Here, we assess how driving with this same sleep restriction and BACs (av 40mg and 28mg alcohol/100ml blood at beginning and end of drive respectively) respond during the evening circadian rise in alertness. In a 2 x 2 (alcohol versus control drink [double blind] X normal night sleep versus sleep restricted), repeated-measures design, 8 healthy young men drove for 2h from 18:00h, in a real-car simulator, on a monotonous, simulated highway. Driving impairment (lane drifting), subjective sleepiness, and EEG measures of sleepiness were recorded. While sleep restriction alone produced significant impairments to evening driving and subjective sleepiness, alcohol alone did not. However, alcohol combined with sleep restriction significantly worsened all indices, although, this was less than that found for afternoon driving with identical interventions. Whereas low BACs may not affect driving in normally alert drivers in the early evening, the addition of moderate sleep restriction still produces a dangerous combination. Probably, there is no "safe" level of alcohol intake for otherwise sleepy drivers, at any time of the day.

Barrett P.R., Horne J.A. & Reyner L.A. Afternoon sleepiness and driving impairment when breath alcohol levels have fallen to zero. Human Psychopharmacology, 2004,19; 421-423.

Epidemiological findings point to very low blood alcohol levels heightening the risk of sleep-related fatal road crashes. This was further assessed using a full sized interactive car simulator. Twenty, sleep restricted healthy young men underwent a 2h simulated afternoon monotonous drive, having previously consumed nil alcohol or 3 units >90min previously, and having near-zero BrACs at the start of the drive. In a repeated measures, double blind, balanced design, driving performance, subjective sleepiness and EEG were monitored throughout. Compared with nil alcohol, the alcohol condition initially increased sleepiness-related driving impairment. However, this was not mirrored by subjective sleepiness or EEG. An unexpected reversal (i.e. improvement) in driving impairment occurred with the alcohol group, in the second hour of the drive. This was supported by a trend for improved subjective alertness. Alcohol continued to interact with sleepiness-related driving impairment after BrACs had reached zero. However a lack of subjective perception of increased sleepiness, at this time, further points to the dangerous combination of even modest alcohol intake and sleepiness, and confirms the road crash findings. BrACs are a poor guide to driver impairment.

Barrett P.R., Horne J.A. & Reyner L.A. Sleepiness and low alcohol intake in women drivers: more impaired but better insight than men? Sleep, 2004, 27; 1057-1062.

We have previously shown that low blood alcohol concentrations (BACs - around half UK/ most US legal driving limits) exacerbate moderate sleepiness (night-sleep restriction to 5h) and markedly impair driving ability in young men. There are distinct physiological gender differences in the absorption, metabolism and central nervous system (CNS) effects of alcohol, therefore we replicated this earlier study, with women, using similar BACs to provide a comparison. Using a 2 x 2 (Alcohol vs. control, normal sleep vs. 5h sleep) repeated measures, counterbalanced design there was a 2h drive from 14:00h in an instrumented car on a simulated highway. Driving impairment (lane drifting), subjective sleepiness and EEG measures of sleepiness. Sleep restriction significantly worsened driving performance and subjective sleepiness as with men. Surprisingly, unlike men, women showed no apparent adverse effects of alcohol alone on these indices; seemingly, they compensated. However, alcohol's effects were profound when combined with sleep restriction, nevertheless women were aware of this enhanced sleepiness, unlike men. After alcohol the EEG showed increased beta activity, an effect not seen in men, indicating a differential pharmacokinetic effect of alcohol on the CNS and/or compensatory effort. Debriefing questionnaires indicated women were aware of the varying risks of driving under these different conditions. Legally "safe" BACs markedly worsen sleepiness impaired driving in women. However, they seem aware of their impaired driving, and able to judge the degree of risk entailed. Such an attitude may contribute to the lower incidence of sleep and/or alcohol related crashes in women compared with men.

Philip, P., Vervialle, F., Le Breton, P., Taillard, J. & Horne, J.A. Fatigue, alcohol and serious road crashes in France - a factorial study of national data. British Medical Journal, 2001, 322: 829-830.

France has a high rate of road traffic crashes. Although driver fatigue may be an important factor, it has not been investigated in France, and no comparisons have been made with alcohol related crashes. We investigated the role of fatigue in serious road crashes using the French national database.

3) Drivers' awareness of their sleepiness

Reyner L.A. & Horne J.A. Falling Asleep Whilst Driving: Are Drivers Aware Of Prior Sleepiness? International Journal of Legal Medicine. 1998, 111: 120-123.

Falling asleep at the wheel is a common cause of road accidents, but little is known about the extent to which drivers are aware of their sleepiness prior to such accidents. It is an area with medico-legal implications. To simulate this situation 28 healthy young adult experienced drivers, sleep restricted the night before drove for 2 h in the afternoon in an interactive real-car simulator incorporating a dull and monotonous roadway. Lane drifting, typifying sleepy driving, was subdivided into minor and major incidents, where the latter was indicative of actually falling asleep. A distinction was made between the subjective perceptions of sleepiness and the likelihood of falling asleep which drivers reported separately. Increasing sleepiness was closely associated with an increase in the number of incidents. Major incidents were preceded by self-awareness of sleepiness well beforehand and typically subjects reached the stage of fighting sleep when these incidents happened. Whilst the perceived likelihood of falling asleep was highly correlated with increasing sleepiness, some subjects failed to appreciate that extreme sleepiness is accompanied by a likelihood of falling asleep. It was not possible for our subjects to fall asleep at the wheel and have an "accident" without experiencing a sustained period of increasing sleepiness, of which they were quite aware. There is a need to educate at least some drivers that extreme sleepiness is very likely to lead to falling asleep and a high accident risk.

Horne J.A. & Baulk S.D. Awareness of sleepiness when driving. Psychophysiology, 2003, 41: 161-165.

The extent to which sleepy drivers are aware of sleepiness has implications for the prevention of sleep-related crashes,especially for drivers <30y who are most at risk. Using a real car interactive simulator, we report on EEG, subjective sleepiness and lane drifting (sleepiness related driving impairment) from 38 sleep restricted, healthy young adults undergoing non-treatment control conditions from three (unpublished) investigations using the same experimental protocols for assessing various drinks intended to alleviate sleepiness. Participants drove 2h mid-afternoon under monotonous conditions. For all studies, subjective sleepiness and EEG activity indicative of sleepiness were highly correlated, with both changing concomitantly, along with lane drifting. Drivers had knowledge of their physiological sleepiness. There were indications that sugar content of these drinks may additionally affect sleepiness.

Horne J.A. Misperceptions about unforewarned "sleep attacks" when driving. British Medical Journal, 2002, 325: 657.

Around half of all sleep related road crashes are caused by healthy adults aged under 30y [1]. We investigate many such crashes and find that most drivers genuinely deny having fallen asleep, and the evidence has to come from elsewhere. Other research [2] shows that momentary sleep can go unnoticed. Moreover, these drivers usually deny knowledge of prior sleepiness, even those admitting to having fallen asleep. Ostensibly, it was an "unforewarned sleep attack". Thus, claims [3] that Parkinson's disease (PD) drivers are prone to unforewarned sleep attacks, should be treated cautiously. Few of us can remember clearly how sleepy we were last night or when this sleepiness began. Recollection is worse if one thinks back further. Hence, it is usually pointless asking drivers to recall whether they were sleepy before their crash. This is no excuse for falling asleep at the wheel. By monitoring healthy, sleepy drivers under safe driving conditions, we [4] find they have excellent on-line insight into their sleepiness. Nevertheless, even when sleepiness is plainly self-evident (eg. opening the window), some drivers maintain they are competent to drive, when this is clearly not so. Fortunately, few of the "sleep attacks" in PD drivers result in injury. Had these occurred spontaneously when driving, then more injuries would be expected. Better comparisons should be made with healthy ageing effects and with other PD drivers not having such attacks. Attribution of these attacks to dopamine agonists, because drug withdrawal alleviates them, overlooks the point that drivers who fall asleep at the wheel are careful not to allow this to happen again, anyway. Unknowingly, many PD patients have sleep disturbances [5]. These cause excessive daytime sleepiness that is often overlooked by the physician. Patients' own opinions about their sleep quality are unreliable- eg. people with debilitating sleep apnoea often claim to sleep well. Although patients with sleep disorders may experience more rapid onset of sleepiness when driving, healthy individuals falling asleep at the wheel usually do so because they are sleep deprived. To the extent that sleep loss is similar for both, then manifestation of the sleepiness may well be similar. These patients may be at no greater a driving risk than the foolish young man who drives without sleep in the small hours of the morning. One treatment for both is better education about the dangers of driving when sleepy.

4) Evaluation of countermeasures to driver sleepiness

Reyner L.A., Horne J.A. & Flatley D. Effectiveness of UK motorway services areas in reducing sleep-related and other collisions Accident Analysis and Prevention 2010 (in press)

In the UK, motorway service areas (MSAs) are believed to be helpful in reducing sleep (‘fatigue’) related collisions (SRCs), however, their actual effectiveness has yet to be evaluated. During a 2-3 year period, and over two sections of UK motorways comprising 14 MSA sites, assessments were undertaken of all fatal and injury road traffic collisions (RTCs), especially SRCs. Analyses examined whether there was: i) a reduction in collisions 16 km beyond MSAs compared with the same distance beforehand; ii) accumulation of collisions with increasing inter-MSA distances. Within the 16km regions there was a non-significant fall in all RTCs from 355 before MSAs, to 304 afterwards. However, the 22% decrease in SRCs (108 versus 84) was significant. Cars comprised the greatest reduction in SRCs possibly attributable to a MSA. Including and beyond these 16km regions, there was no correlation between inter-MSA distances and accumulated RTCs or SRCs, (n=682 of which 181 were SRCs [26%]). There were obvious differences between MSAs in all these respects. Of the 23 fatal RTCs, 17 were SRCs. Whilst SRCs had their greatest daily incidence between 02:00h and 06:00h, as expected, MSAs seemed to have their least beneficial effect on SRCs during this vulnerable period, which is a matter for concern.

Horne J.A. & Reyner L.A. Driver sleepiness: comparisons of practical countermeasures - caffeine and nap. Psychophysiology, 1996, 33, 306-309.

Sleepy drivers should "take a break", but the efficacy of feasible additional countermeasures that can be used during the break is unknown. We examined a shorter than 15 min nap, 150mg of caffeine in coffee, and a coffee placebo, each given randomly across test sessions to 10 sleepy subjects during a 30 min rest period between two 1 hr monotonous early afternoon drives in a car simulator. Caffeine and nap significantly reduced driving impairments, subjective sleepiness, and electroencephalographic (EEG) activity indicating drowsiness. Blink rate was unaffected. Sleep during naps varied, whereas caffeine produced more consistent effects. Subjects acknowledged sleepiness when the EEG indicated drowsiness, and driving impairments were preceded by self-knowledge of sleepiness. Taking just a break proved ineffective.

Reyner L.A. & Horne J.A. Caffeine combined with a short nap effectively counteracts driver sleepiness. Psychophysiology, 1997, 34, pp 721-725.

Previous research has shown that caffeine and a <15 min nap effectively and separately reduce sleepiness in drivers for 1 hr. In the present study, we examined in 12 sleepy individuals the treatments combined, taken during a 30 min break, prior to a longer (2 hr) continuous monotonous afternoon drive in a car simulator. Non-nap comparisons were 200mg caffeine only and placebo. For placebo, driving incidents, subjective and electroencephalographic measures of sleepiness all reflected a mid-afternoon peak. This peak was significantly reduced by caffeine and eliminated by the combined treatment, which reduced incidents to 9% of placebo levels versus 34% of placebo levels for caffeine alone. Naps comprising "nonsleep dozing" were still effective.

Reyner L.A. & Horne J.A. Early Morning Driver Sleepiness: Effectiveness of 200mg Caffeine. Psychophysiology, 2000, 37: 251-256.

Sleep-related vehicle accidents are prevalent early morning, especially in younger drivers. In two independent studies following a night of either restricted or nil sleep, young experienced drivers drove for 2 hr (06:00-08:00h) continuously in an immobile car on an interactive, computer-generated, dull, and monotonous roadway. This exercise followed ingestion (at 05:30h) of 200 mg caffeine (=2-3 cups coffee) versus placebo, counterbalanced, double blind. Driving incidents (lane drifting), subjective sleepiness, and 4-11 Hz electroencephalogram (EEG) activity were logged. In Study 1 (sleeping 00:00-05:00h), caffeine significantly reduced incidents and subjective sleepiness throughout the 2 hr drive, and EEG power for the second 30-min period. In Study 2 (no sleep), sleepiness affected all measures profoundly, and driving was terminated after 1 hr. Nevertheless, caffeine reduced incidents significantly for the first 30 min and subjective sleepiness for the hour. This caffeine dose, feasibly taken via coffee, effectively reduces early morning driver sleepiness for about 30 min following nil sleep, and for around 2 hr after sleep restriction.

Reyner. L.A., & Horne, J.A. Efficacy of a functional energy drink in counteracting driver sleepiness. Physiology and Behaviour, 2002, 75:331-335.

Driver sleepiness is a major cause of serious road crashes. Coffee is often used as an effective countermeasure to driver sleepiness. However, the caffeine levels in coffee are variable, whereas certain proprietary "functional energy drinks" (FEDs) contain known levels of caffeine (and other ingredients). We investigated the effectiveness of a well-known FED in reducing sleepiness in drivers. Twelve healthy young adults drove an instrumented car simulator, between 14:00h and 17:00h. Their sleepiness was enhanced by sleep restriction to 5h the night before. Following a pre-treatment 30 min drive, and at the beginning of a 30 min break, participants were given double blind 250 ml FED (containing sucrose, glucose, caffeine [80mg]), taurine, glucuronolactone and vitamins) Vs a control drink with the same volume, and tasting the same but without caffeine, taurine and glucuronolactone. Two hours continuous driving ensued. Lane drifting, subjective sleepiness and the electroencephalogram (EEG) were monitored throughout. Compared with the control, the FED significantly reduced sleep related driving incidents and subjective sleepiness for the first 90 minutes of the drive; there was a trend for the EEG to reflect less sleepiness during this period. It was concluded that the FED is beneficial in reducing sleepiness and sleep-related driving incidents, under conditions of afternoon monotonous driving following sleep restriction the night before.

Horne, J.A. & Reyner, L.A. Beneficial effects of an "energy drink" given to sleepy drivers. Amino Acids, 2001, 20: 83-89.

500ml of a glucose based "energy" drink versus a control without the active ingredients (caffeine, taurine, glucuronolactone) were given double blind to 11 sleepy participants driving an interactive real-car driving simulator. Lane drifting and a secondary task (reaction time) were measured for two hours post treatment. The energy drink significantly improved both indices, particularly for the first hour.

Anderson C., & Horne J.A. A high sugar content, low caffeine drink does not alleviate sleepiness but may worsen it. Human Psychopharmacology, 2006, 21: 299-303

Although the ingestion of high levels of glucose might have a short acting alerting effect, there is evidence of an ensuing enhancement of sleepiness in people already sleepy. Some 'energy drinks' contain large quantity of sugars. We compared 250 ml of a well known 'energy drink' (42 g sugars, containing a low [30 mg] level of caffeine for 'flavouring') with a nil sugar nil caffeine, similar tasting control. These were given a week apart, in a repeated measures, double blind, balanced design, to 10 participants sleep restricted to 5h the prior night. They had a light lunch, consumed a drink at 13:50h, and 10 min later underwent 3 x 30 min consecutive periods at a reaction time task (the Psychomotor Vigilance Test), separated by 3 min breaks when self-ratings of sleepiness were made. The energy drink did not counteract sleepiness, and led to slower reaction times and more lapses during the final 30 min session, around 80 min after consumption.

Reyner L.A. & Horne J.A. Evaluation of "In Car" Countermeasures to Driver Sleepiness: Cold Air and Radio. Sleep, 1998, Vol. 21, No.1, pp46-51.

The efficacy of putative "in-car" countermeasures to driver sleepiness is unknown. Sixteen young adult drivers within the normal range for the Epworth Sleepiness Scale (ESS), had their sleep restricted to 5 hours the night before, and drove an interactive car simulator in the afternoon for 2.5 hours, under monotonous conditions. After 30 minutes of driving they were exposed to: (1) cold air to the face (AIR) from the vehicle's air conditioning vents, (2) listening to the vehicle's radio/tape (RADIO) according to subjects' choice, or (3) NIL treatment. The active treatments typified those experienced under real driving conditions. Drifting over lane markings were "incidents". EEgs were recorded and spectrally analyzed in the alpha and theta range. Subjects responded to the Karolinska Sleepiness Scale (KSS) every 200 seconds. Overall, RADIO and AIR had no significant effects on incidents, although there was a trend for RADIO to reduce incidents, particularly during the first 30 minutes, when AIR also had some effect. KSS scores were significantly lower for RADIO for most of the drive, whereas AIR had only transient and non-significant effects. The EEG showed no significant effects of the active treatments. Compared with other countermeasures such as caffeine and a brief nap, which we have previously shown to be more effective (using the same equipment and protocols), AIR and RADIO are at best only temporary expedients to reduce driver sleepiness, perhaps enabling drivers to find a suitable place to stop, take a break and avail themselves of caffeine and a nap.

Baulk, S.D., Reyner L.A. & Horne J.A. Driver Sleepiness: Evaluation of Reaction Time Measurement as a Secondary Task. Sleep, 2001, 24(6): 695-698.

The application of reaction time (RT) as a secondary task to determine sleepiness in drivers is of increasing interest, but is a problematic area. We assessed the extent to which RT reflected this sleepiness, and/or otherwise affected driving behaviour in sleep restricted, moderately sleepy people. They drove a real-car interactive simulator for two, two hour afternoon monotonous drives, with and without RT (counterbalanced). Simple auditory RT was used, with a semi-random inter stimulus interval averaging two and a half minutes. Lane wandering(driving "incidents"), subjective and EEG measures of sleepiness were obtained. For both conditions all three indices changed significantly during the course of the afternoon circadian "dip". However, this was not reflected in RT, which remained relatively stable. Nevertheless, RT provided more "stimulation" for the sleepy driver, and significantly reduced subjective sleepiness, with a trend for fewer incidents and a more alert EEG. Possible reasons for the disparity in sensitivity between RT and the other measures are discussed. Under this experimental protocol, RT did not provide a useful guide to driver sleepiness; it was merely a mechanism for increasing task load and reducing monotony. The drivers' own insight into their sleepiness had more validity as a tool for assessing sleepiness.