https://en.m.wikipedia.org/wiki/I,_Libertine
I stumbled across this story and became interested when I read about its reveal in the Wall Street Journal. It got me thinking that in an age when television was just taking off, and radio had a distinctive nighttime aesthetic as compared to daytime radio, that unlike today, people who worked late night hours might become something of a distinct social group as those who slept through the day would miss much of the media and entertainment of the daylight hours.
For Jean Shepherd, the 'Night People' he talks about would be the ones listening to his late night shows on WOR. Mostly students and others who might enjoy his humorous monologues, and have to time to listen - what you might call the real 'Night people' (the ones working late hour jobs) would be at work or out in clubs at the time, and not able to listen in.
Shepherd wrote an article for Mad Magazine in 1957 - available here - in which he gives his description of the 'Night People', of which this is the introduction :
In the language of "Day People", I suppose "Night People" may be called many things. Like "soreheads", "wise-guys", "eggheads", "long-hairs", "outsiders", etc. What-ever they're called, the fact remains they're a genuine phenomenon. They're the people who refuse to be taken in by the "Day World" philosophy of "Creeping Meatballism".
'Meatballism' was Shep for what you might call the 'squares', the normal rat-race world.