“Back in the 1940s people were sleeping on average just a little bit over eight hours a night, and now in the modern age we’re down to around 6.7, 6.8 hours a night,” says Matt Walker, professor of neuroscience and psychology at the University of California, Berkeley.

“So that’s a staggering loss of sleep within the space of just 70 years, we’re now almost at the stage where we’ve lopped off 20% of that.”

For adults, the modern world is full of things which reduce sleep. Caffeine, which keeps us awake. Alcohol, which fragments our sleep and suppresses dreaming. And although we’ve improved the conditions for sleeping, with everything from better mattresses to smoke-free homes, our controlled environment may also have created problems, Walker says.

“One, which people may find surprising, is central heating and central air conditioning. So, when the sun sets, temperature drops dramatically and when the sun rises it starts to pick back up. Our bodies expect that beautiful thermal lull. And what we have done is dislocate ourselves from the natural ebb and flow that tells us when it’s time to sleep and actually helps us get to sleep.”