The dictionaries are uncertain about yom. Rabbinic sources offer various suggestions.
One view links the word with mayim, water, “since light comes from the water”, possibly understanding light as a wave motion. Another view sees yom as a derivative of hum or him, meaning to move.
Samson Raphael Hirsch, less grammatically, suggests a connection with kum, to arise, since day is the time when people stand up; by way of contrast, Hirsch links lay’lah, night, with a root that means to lie down.
Driver says, “God designed the distinction (between light and darkness) to be permanent, and therefore stamped it with a name.”
There are things to do during the day. The Psalmist says, “The sun rises… man goes forth to his work” (Psalm 104:22-23). Daytime is when one has work to do and can see how to do it.
Hillel Zeitlin said, “Each day in which no good was done returns to its Creator in disgrace. Each day in which good was done weaves a garment for the human soul.”
It is a waste of the daylight hours to spend them in night-time activities; Rabbi Dosa ben Horkinas said that morning sleep “drives a person out of the world” because valuable opportunities for work and study are being lost (Avot 3:14).
The night is the time for sleep, in order to be refreshed for the day ahead. To be unable to sleep is a major problem, but one of the ways of facing the problem is to try to restore the day-night distinction.
There are of course those whose occupation requires night-time work, but this should not be regarded as normative.