An exclusive test carried out by What Car? in simulators at the Transport Research Laboratory compared the ability of a tired driver with a drunk driver.
Two drivers were put through a series of tests.
Driver 1 was not allowed to sleep until 3am and Driver 2 consumed 12 units of alcohol over the course of a night.
They were both then asked to negotiate a similar route that included motorways and A-roads.
When 25% over the legal limit, the drunk driver's ability to hold a steady distance from a car in front dropped by 25%, while his ability to keep to the centre of a lane dropped 23%.
At 2am, the sleep-deprived driver was 39% less able to maintain a steady distance from a car in front, and his capacity to drive within a lane suffered by 37%.
The drink driver did prove to be the worse driver overall.
When twice over the legal limit he was 49% worse at maintaining a steady distance and 42%worse at keeping to his lane.
Researchers also found that 40% of drivers admit to falling asleep at the wheel.
Steve Fowler, What Car? group editor, said: ‘It’s not surprising to learn that drink seriously affects driving ability, but it is worrying to discover that a sober person’s driving ability can suffer more when they are tired.
‘Our message to drivers is simple: don't drink and drive and don’t drive tired. If you are feeling drowsy, pull over in a safe location, lock the doors and have a nap – better to arrive late than not at all.'