Steve states that the best time to cry is at night because, normally, someone is being attacked and if the noise of the attack is loud enough no one would be able to hear you cry. Steve is fearful that if someone hears you cry that you will be seen as weak and that you will be the next one to get attacked. Steve knows that jail/prison is a place full of violence, that he does not being here, and showing any weakness will cause him problems with the other inmates.
The reason why it was easier to cry at night because then you wouldn't be the next one to scream, and most would never help others. Steve and other criminals who are in this position decide that while someone else gets pain they can let go of theirs, and feel free from judgement.
because the lights are out and someone is being beaten up and screaming for help, no one will hear you cry