Robin goes out at night rather than during the day (presumably she sleeps all day since she’s out all night) because the darkness helps Robin remain somewhat anonymous even while she explores the darker parts of herself with other people. Under cover of darkness, Robin can be more herself (for instance, it was at night that Robin was first able to express that she didn’t want a baby). For this reason, Nora starts going out at night too—she hopes she’ll be able to learn more about Robin this way, at least the parts that Robin tries to keep hidden during the day. Matthew thinks Nora is “dismantled” because Nora’s losing her sense of identity and self in her obsession with Robin.