Well, only about 42% of the population works. So we are already below half the population. Of the working population, there might be actual stats for who works off hours, but you can make a guess. Because most people are awake during the day time and are not up all night, most of the service industries are heavily loaded with workers during these times. Restaurants get busy in the evenings, so their staffs have to reflect that. Some businesses are around-the-clock operations. As has been mentioned, Walmart, some industries, hospitals, etc. But from the working population, this isn’t a huge amount. These industries usually have a smaller staff on back shifts because of the needs.
So a total percentage of the population that works nights and evenings is a swag at best. I’d put it somewhere around 10%.