Kevin Smith actually worked at the convenience store! He would clock in at 6 am and finish at 11 pm, then shoot straight all night. They did this for 21 nights! Then, the super-dedicated Smith would take a quick 1-2 hour nap before returning to work.
This is the reason why the shutters are down in the movie – they couldn’t afford to light the set like the way daytime light would look, and worked the closed shutters into the plot.