They are attempting to make the inmates feel less than human. By taking away everything that reminded them who they were, they begin to forget their identity or begin to accept this new identity. When they appear less than human, it also makes them easier to kill.
Part of this is by design on the Germans part. People are afraid of the dark and the dark can be disorienting, especially when you are in a new place. However, the writer probably emphasizes this because he feels that this is a dark place in his life and for the people around him. It feels to him that there is no hope, no light. Everything at the concentration camp is centered around death, around darkness.
The germans are trying to dehumanize the inmates by taking what they hold close and cherish. By tattooing them it signifies they are less than human, and cutting their hair can act as a form of dominance and making all the inmates similar and less human. Most happens at night because it was a dark time and the only time they really had to themselves was at night.
