Day-And-Night Cycle

Overview
The Day-And-Night Cycle is the central focus of the After Dark expansion pack. It is the periodic change of the time of day. When you start a city, you will automatically start in the day and then progress to night. A full cycle is about 113 in-game days, which is roughly equivalent to 9 minutes and 35 seconds at 3x speed.

You are able to turn this off in the option menu. Doing so will not affect achievements whatsoever.