“THE notion that nothing good happens after midnight does not seem to apply to times of birth. Around the world the peak hours for vaginal births that have not been induced by drugs fall between 1am and 7am; the numbers then dwindle throughout the rest of the day. This has led many scientists to believe that giving birth during the early morning offers some sort of evolutionary advantage, perhaps gained long ago when hunter-gatherer mothers and their infants would benefit from having their group reunited during the small hours to help with care and to defend them against any predators.
The problem with this theory is that almost all the information on the timing of human births comes from modern, urban settings, such as clinics and hospitals, which could produce artificial conditions that skew the variation in timings. Not so, it turns out. As Carlye Chaney of Yale University shows in the American Journal of Physical Anthropology, early-morning births are common to communities with both modern and traditional lifestyles.”
(click link at top to read this fascinating article on economist.com)