The sun, or rather, the two binary stars were just peeking into the horizon when dinnertime came. Well, this phenomenon was irrelevant though since the Core of this inside-out planet constantly gave off a bright light that illuminated the entire world. Nighttime is not a question here as it doesn't even exist, but they were able to tell the time through their clocks, which in turn gave them the notion of daytime and nighttime even though their presence wasn't there.
It was only recently that nightbulbs were invented by none other than Luxor Camberwell. And true to its name, it provided a nightly atmosphere in an enclosed space no matter how bright it is outside— the very opposite of a lightbulb.
Every household has one, and they use it for the other half part of the day unless, of course, they needed the light for something.
