Stevie writes that night time is the best moment to cry because it prevents the other inmates from knowing that you are scared.

In the opening to Monster, Stevie depicts a world where people are hunted. Those who are scared and weak are seen as primary targets:  

The best time to cry is at night, when the lights are out and someone is being beaten up and screaming for help. That way even if you sniffle a little they won't hear you. If anybody knows that you are crying, they'll start talking about it and soon it'll be your turn to get beat up when the lights go out. 

Stevie's writing shows that the night time is when a prisoner is physically abused by other prisoners.  In the midst of this abuse, it is easier to cry because no one will hear you.  When the other inmates hear someone crying, it places a target on them and they become victimized. Stevie's writings convey the fear intrinsic to prison.  Whenever someone transmits this fear, they become targeted.  It is for this reason that nighttime is the best time to cry. As someone else is being abused, it provides the cover to cry and be scared because no one will hear it.