But perhaps there is a positive twist—in the form of a mid-afternoon nap backed by science. Researchers who have looked at cultures that remain completely untouched by electricity, such as the Hadza of Tanzania, found that, especially in the summer, these groups tend to sleep biphasically: packing in six hours a night, and then a few hours again in the afternoon. This begs the question, Walker says, of whether human beings should stay awake for sixteen hours straight. In fact, everyone goes through an afternoon lull. Biologists have actually been able to measure this physiological dip in our alertness—what science calls a postprandial dip—through changes in our metabolic system, as well as adjustments in our brain waves, and in our cognitive reaction times. The universal cognitive pause means we might benefit from a nap around this time. “Perhaps human beings need to sleep biphasically for truly optimal performance, though that’s still unclear,” Walker says.