Lights Out is a routine present in all prisons. It typically begins at night, though some prisons have irregular schedules (such as USS. Anomaly) . During this period, all cell doors will be locked with a yellow key, and are opened at the end of Lights Out.
If you are not in your cell by 1 minute (1 in-game hour) after the start of Lights Out and you are caught outside your cell while wearing any variant of an Inmate Outfit, you will be sent to Solitary and it will become morning. If the guards see that you are not present in your cell and have no Bed Dummy on your bed, you will also be sent to solitary (no matter what outfit you are wearing.)