Eight hours of sleep—doctor's orders, right? Perhaps not. A number of recent studies have pointed to a human tendency to sleep in two sessions of about four hours each, with an hour or two between, the BBC reports. A researcher in the 1990s put subjects in darkness 14 hours a day in a month-long study. Four weeks into the experiment, the subjects were regularly sleeping for four hours, waking for an hour or two, then sleeping four more hours.
It's not just a modern thing, it seems: A book published a few years ago finds some 500 references to such a sleeping pattern throughout history, in everything from fiction to diaries. During the waking session, people might read, smoke, or visit friends. Fifteenth-century prayer books even listed prayers specifically designed for the hours between sleep. But with improvements in lighting over the centuries, the idea of the two sessions faded; by 1920, it was gone. Still, "for most of evolution we slept a certain way," says a sleep psychologist," so waking up in the middle of the night shouldn't necessarily be worrying—it's "part of normal human physiology."