Cycles refer to the day/night cycles in Oxygen Not Included. Each cycle lasts for 600 seconds. 525 of that is daytime and 75 seconds is night. This time cycle is represented by the slowly rotating meteor in the top left corner of the player's UI. Cycles can be paused or sped up by using the controls next to the meteor. Players can also pause the cycle by pressing the spacebar (PC). The speed controls are 1x, 2x, and 3x.
Duplicants will perform their work tasks during the daytime and sleep at the time the schedule is set to. However, there are nighttime issues that can cause the Duplicants to lose sleep. While a full night's rest is not required to restore stamina, a restless night can cause additional stress. Things like hunger and a full bladder will wake them up. Stats like Flatulence and Loud Sleeper can cause Duplicants to wake others up and add to their stress. If there are any red alerts, Duplicants will not sleep.
Nighttime is also when Hatches come out. They will be actively seeking food during the night, and find a place to dig back into to hide during the day.