The reporting of this research seems to misinterpret the results a bit. It's important to keep in mind that North American studies are based on self-reporting, and it has been demonstrated that people self-report time in bed, rather than actual sleeping time.
Thus, the main result of the study is that Americans and hunter-gatherers sleep about the same on average, and that the amount of actual sleep associated with negative health outcomes in North America is less than 4 - 6 hours per night (as opposed to 7 - 8 hours).
The study also shows that technology and modern lifestyles do not, on average, disrupt North American sleep cycles. Further, the study indicates that humans do not naturally sleep according to dark and light, but rather to the ambient temperature. Finally, it suggests that humans may not naturally engage in biphasic sleep, although this could be because of the daily temperature cycles in East Africa and the Southern Andes.