Drowsy Driving Impairs You Like Alcohol — Stop Somewhere Safe
Tiredness slows you down much like alcohol. In lab studies, 17–19 hours without sleep impaired reaction time and coordination on some tests about as much as a blood alcohol level of 0.05%, and around 24 hours awake about as much as 0.10% (Dawson & Reid, 1997; Williamson & Feyer, 2000). The most dangerous part is microsleep: you can drift off for a few seconds without noticing, and at 100 km/h the car covers about 28 metres every second.
If you feel sleepy at the wheel, first stop somewhere safe — a service area or car park, not the hard shoulder (UK Highway Code, rule 91). Once stopped, a caffeinated drink and a short nap of about 15 minutes help for a while: in a simulator study they cut driving errors in sleepy drivers, while a break alone did not (Horne & Reyner, 1996). After the nap, give yourself a few minutes to wake up properly — right after sleep you can be groggy for a while (sleep inertia, NIOSH, US). The alarm going off does not mean you are fit to drive: if you still feel sleepy, don't set off. It is a stopgap, not a substitute for sleep — if sleepiness returns, stop again or end the drive. There is no good evidence that an open window or loud music keeps you safe.
More: how caffeine works and the short nap.
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