What is the purpose of the many repetitions of "night"? to reflect her memory at night time?
Almost every other chapter is entitled "night" because that is when Offred can be alone and be reflective about her life and the experiences and situtations that she must face each day. It is also a time when she can remember her life before this revolution and complete change in the status of women. She can be most truly herself, not a Handmaid. During the day her speech is restricted to by the headwear of the handmaids. Her actions are restricted. She is restricted in where she can go and with whom. At night is when she has opportunities to go back to the past -- perhaps in her memory, but also in her interactions with her Commander. He brings her lotion and she gets to put it on; they play Scrabble; she reads old women's magazines - these are all activities she took for granted before, but now she savors them because they are forbidden. She also, eventually, carries on her affair with Nick in the night. Her time at night is her own. She feels less like the person the new regime has required her to be and more the person she knows, deep down, that she was and still is.