Oh, I'm not the best placed for this.
From what I know, the day-night cycle is made thanks to the cloud texture: unlike the atmosphere texture, the cloud texture loops around the planet, and you can parameterize its speed in the planet file. By using a texture that has a day lighting alternated with a night lighting and setting it as the cloud texture you can create a day-night cycle.
Other planet editors may have a more accurate answer. Maybe have a look at Gurren's ACSS, I believe it has such a feature, you should be able to take inspiration from it.