For much of early modern Europe, sleep didn’t seem to have followed the single, unbroken pattern we use today. A large body of historical evidence points to many people sleeping in two stretches, with ...

The amount of sleep a person needs recalibrates over time, and circadian rhythms change with age. The 16-year-old who stayed up until midnight may become a 36-year-old who gets drowsy by 10 p.m. At 70 ...