Prior to the 17th century, people held fast and true to a segmented sleep routine: sleep for four hours, wake for a bit to smoke tobacco or meditate on dreams, and then fall back into a second four-hour sleep. What happened? Streetlights were invented—which made the night a far friendlier place. But street lights also spurred a shift in perspective: If you could stay out later, then time suddenly became a greater consideration. Thus the dawn of our efficiency-obsessed culture began. But it turns out that a consolidated, eight-hour sleep is completely unnatural—and over time, we’ve managed to completely circumvent the body’s natural circadian rhythms.