A mutation of our
inherent tendencies. Most probably instigated by the disappearance of
agricultural communities, wherein, I presume, people went to bed early in order get up
bright and early for the next day's necessary
chores.
With the disappearance of self-sufficiency and newfound dependence on feudal lords or employers, people may have viewed nightlife as a form of existential protest. Time embezzled during the day would be reclaimed at night, and thus man would also become a nocturnal being due to the social constraints of the day, albeit when he could manage it.
One must not forget the role alcohol, sex, drugs, and loud music plays in this augmented existence. These serve the subject further in revenging himself upon the creatures of the day, which by now he/she despises and views as the binary opposites of his desires.
Sound like my two cents?