The Friday File: After closely observing three pre-agricultural groups, one from Tanzania, one from southern Africa and one from Bolivia, researchers learned that these peoples averaged 6.5 hours of sleep/day. Moreover, the groups stayed awake for an average of 3.3 hours after nightfall, rarely napped in the afternoon and barely suffered from insomnia. This suggests that coffee, electricity and modernity haven’t, contrary to popular belief, led to systematic sleep deprivation.