Some are ... some aren't. If the pump lights are lit, the pump is most likely active - however, if the station is dark and the parking lot lights are out ... do you really want to be pumping gas in the dark?
When I travel, I usually have no problem finding stations that are open all hours of the night ... often these are convenience stores - especially name brands like 7/11, Pilot, Wawa, Love's, RaceTrac and RaceWay and many Walmart stores have a Murphy station.