We all know 8 hours is the magic number for a decent night’s sleep. Or is it?

Nobody seems to know where this number came from. In questionnaires, people tend to say they sleep for between 7 and 9 hours a night, which might explain why 8 hours has become a rule of thumb. But people also tend to overestimate how long they have been out for the count.

According to Jerome Siegel, who studies sleep at the University of California, Los Angeles, the 8-hour rule has no basis in our evolutionary past – his study of tribal cultures with no access to electricity found that they get just 6 or 7 hours. “And those people are pretty healthy,” adds Derk-Jan Dijk at the University of Surrey, UK.