Why Do Humans Sleep in 8-Hour Cycles?
For much of recorded history, humans have actually slept eight hours, but in two distinct phases of approximately four hours each. As scholar Roger Ekirch uncovered through his exhaustive historical study of literature, art, and diaries, people would once head to bed when it got dark, sleep for four hours, wake for a while, and slide into a “second sleep” for another four hours.
As artificially illuminating the night sky became more affordable, life—and soon sleep habits—changed. Fifty European cities introduced tax-supported street lighting by 1700 and made it safe and socially acceptable to move about publicly after dark, a time of day that had previously been considered the domain solely of prostitutes and other suspicious characters. In 1787 fashioned the first mechanical alarm clock specifically to rouse him for work, it marked a new understanding of the relationship between sleep and labor.