Interactions between moderate alcohol consumption and sleepiness : the effect on driver performance
Both alcohol and sleepiness are known to be major contnbutors to road traffic accidents m the UK. There has been much debate on whether the current legal blood alcohol concentration (BAC) limit for driving (008%) should be lowered to 005%, like several other countries in the European Union. The present limit may be sabsfactory when a driver IS fully alert, however the pressures of today's society mean that an mcreasmg number of people may be sleep deprived. The consequences of a sleepy person drivmg after drinking a current legally acceptable amount of alcohol have not been fully investigated. An initial literature review idenbfied specific areas that needed to be investigated throughout this programme of work The research took the form of a "hfelike" scenario, with only moderate sleep restnction (5h in bed at night) and moderate alcohol consumpbOn, producmg BACs of approximately half the UK legal driving limit The drive, on a Simulated dual carriageway, lasted for 2h and was very monotonous The research programme was split into four main areas (i) young men (the most at risk group of drivers for sleep related crashes) driving in the afternoon (a time when the number of sleep related crashes are known to increase), under a 2 x 2 experimental deSIgn, With and without alcohol at lunch-time and with and without the prior night's sleep restncted to 5h, (u) an identical gender comparison usmg young women, (hi) a time-of-day companson using young men, but with the drive and alcohol consumption takmg place m the early evening (a bme of day when we are naturally more alert); (iv) a near-zero BAC, when young men have the same alcohol intake as in (I) but earlier, such that their BACs have reduced to nearly zero before startIng the afternoon drive. Dunng the afternoon circadian trough the driving performance of both men and women is severely impaired when moderate sleep restriction and alcohol consumption are combined Of particular concern, is that men seem to be unable to perceive this greater impairment Women generally appear to have better perception of alcohol impairment, even without sleep loss. Unlike men, women's driving is less impaired by modest amounts of alcohol when they are alert, which seems to be because they know their performance IS affected and thus apply more compensatory effort. On the other hand, their rrnpamnent after alcohol when combmed WIth sleep loss is well in excess of any compensatory effort. Trrne of day also affects imp3lrment after alcohol and/or sleep loss. Driving performance IS generally better during the early evening holtrS, when we are nat\lfally more alert, compared with the afternoon, and for all conditions. Moderate alcohol intake does not impair drivmg performance during the early evening, unlike during the afternoon. However, if combined with sleepiness, mcreased driving impamnent does become apparent during the early evening, although, not to the extent that it does durmg the afternoon. BACs are not a good indicator of alcohol-related driving impairment, especially when combmed with sleepiness. During the afternoon, even when BACs fall almost to zero at the start of a drive, sleepy drivers are still more impaired for the first hour of the drive if they have consumed this modest amount of alcohol at lunchtime An unexpected rebound improvement m dnvmg performance is seen ID the second hour of the drive In non-sleep deprived, alert drivers, these same near zero BAC levels did not affect driving performance or significantly increase subjective sleepiness. Overall the results indicate that, combined WIth modest sleepiness, the current legal dnnk drive limtt (008%) is too htglt Thts outcome supports recent and extensIve findmgs WIth fatal and senous road crashes in France (Philip et al., 2001). During the afternoon, a time of day when people are nat\lfally less alert BACs of less than half this UK limit will impair driving even in non-sleep depnved people If drivers are also sleepy, this combmation produces dangerous levels of Impairment durmg the afternoon; the combination also leads to impairment (but to a lesser extent) in the early evening The research was carried out with only moderate levels of sleepiness and alcohol consumptIon, It is fair to conclude that driving impairment would be greater if the sleep loss was greater and/or BACs were htgher, but just under the legal liemt Greater public awareness is required on the knowledge that driving after consuming any alcohol when tired or sleepy is extremely dangerous.