Not necessarily, now. In the US, it's legally required that if a gas station wants to keep its pumps on throughout the night and into the morning, it must have at least one attendant or employee on standby. This applies even to self-service gas stations. Therefore, if you're driving past a gas station that's completely dark and doesn't have an employee present, it's most likely that they're closed. Besides this, another good way to tell if a gas station is or isn't opened is by checking beforehand with services like Google Maps. Typically, it's common to see big-name gas stations remain open 24/7, while smaller mom-and-pop stations might not stay open overnight.