Why do we sleep? And when we sleep, why do we dream? Do our nocturnal visions serve any fundamental biological purpose? Despite the fact that we spend about a third of our lives asleep and perhaps a sixth dreaming, scientists can offer no clear answer to these questions after many decades of global research.

“The short answer is that we don’t know why we sleep,” admits Yuval Nir, who runs a sleep laboratory at Tel Aviv University. “It is not what the public wants to hear but as a scientist I have to say it.” Vladyslav Vyazovskiy, a neuroscientist at Oxford university, agrees: “Surprisingly, the function of sleep remains elusive.”

We do know that without it we would suffer the consequences. “Sleep is undoubtedly an essential life-support system,” says Matthew Walker of the University of California, Berkeley, and author of the bestselling Why We Sleep. All animals with nervous systems sleep. Sleep deprivation causes disease and in extreme cases may be fatal.