Why does driving make us drowsy?

Nodding off at the wheel is a common experience – and a potentially deadly one. Globally it’s estimated to play a part in 250,000 deaths each year. But while it obviously makes no sense to drive when already half-asleep, research suggests that just being in a moving car can trigger drowsiness in as little as 15 minutes. This July, a team at RMIT University, Australia, published research suggesting it’s the vibrations we experience while driving that trigger sleepiness. Exactly how isn’t clear; intriguingly, however, the frequency of vibration required is around seven cycles per second – similar to that of so-called theta wave activity in the brain, which is linked to falling asleep. Other studies have suggested that the hisslike ‘white noise’ generated by the wheels also plays a role.