In the Cards by George O. Smith

The masked man crept down the corridor stealthily. It was quite dark in the hallway but he knew that it was a synthetic darkness, a matter of temporal convenience, for on a spaceship, time is regulated by the Terran daily cycle of twenty-four hours.

On spacecraft the passenger-sections observe a strict twelve-hour division between sheer brilliance and utter darkness. He estimated that it was a full two hours before light-time, which meant that those couples who preferred to sit and hold hands whilst staring at the rather over-stable aspect of the sky were by now bedded down and asleep.