Got any big plans after "first sleep" tonight? This question might have actually made sense centuries ago, when human sleeping patterns appeared to be much different from those of today. As Zaria Gorvett explains at the BBC, people in the Middle Ages—and perhaps for long before that—commonly slept in what amounts to two shifts. They'd go to bed somewhere between 9pm and 11pm, sleep for a few hours, rouse in the middle of the night for a few hours, then return to bed for the second sleeping shift. The period of wakefulness in the middle of the night was known as the "watch," and people would use it for all manners of productivity—chores, prayers, sex, burglary, you name it.
The story is based on research by historian Roger Ekirch, who noticed matter-of-fact references to "first sleep" or similar phrases in court transcripts from the Middle Ages. He dug deeper and concluded that the practice of "two sleeps"—he dubbed it "biphasic sleep"—was widespread and may have been so for millennia, all over the world. It came to an end for good during the Industrial Revolution, when electric light changed not only our circadian rhythms but cultural norms as well. The story cites modern research suggesting that people gravitate back to the pattern under the right circumstances. Ekirch adds that the discovery might ease the anxiety of those who stress about being unable to sleep through the night—it could be their bodies reverting to an age-old habit. Read the full story. (Read more sleep stories.)