After dark

Nowadays, when a commentator approaches Gen 1, he's keenly conscious of how Gen 1 relates to modern science. Whatever his position on the relationship between Gen 1 and modern science, his interpretation is often has that frame of reference in view. That, however, can lead a modern reader to neglect features in the text that might be significant to an ancient reader. 

I've often discussed the significance of light and dark, day and night, to an audience that existed before the advent of electrical lighting. For instance, dawn, dusk, and night are apt to evoke ominous associations with nocturnal and crepuscular predators. 

I'd like to add a few more observations in that regard:

i) People dream at night. Genesis records several revelatory dreams given to Abimelech, Jacob, Joseph, the Egyptian baker, the Egyptian cupbearer, and Pharaoh. 

Genesis is the only book in the Pentateuch that records revelatory dreams. Is it just coincidental that that's the same book which records God creating day and night? Those are correlative: you can't have one without the other. 

By creating night, God creates the conditions for a mode of revelation. Given the prominence of revelatory dreams in Genesis, that action seems to foreshadow revelatory dreams.

ii) Procreation is a major motif in Genesis. "Be fruitful and multiple." The genealogies. 

Couples normally have conjugal relations at night. That's in part because, before the advent of electricity, sunlight was the primary source of light to work by–especially out of doors. 

In addition, darkness affords a degree of privacy for sexual relations. That's important in cultures where people live in close quarters, viz. one-room huts.

Likewise, there wasn't much to do after sunset. And couples, who might be separated in daytime, according to the division of labor, were back together at night. So one thing leads to another.