Prior to his metamorphosis, Gregor would spend his evenings at home.  His mother tells the manager that he "has nothing on his mind but the business" and that he never goes out at night.  He sits reading the paper or "studying timetables," though sometimes he builds things.  He's used his fretsaw to construct the frame that he has hanging in his room (the one with the picture of a woman from a magazine in it that he gets so defensive of later on).  

Some months after his transformation, the family would leave Gregor's door open so that "lying in the darkness of his room, invisible from the living room, he could see the whole family sitting at the table" and he was able to hear their conversation.  Often, their talk centered on money, and he watched his mother take up sewing and heard about his sister, Grete's, new job as a salesgirl while she learned other skills at night so that she might get a better position in the future.  This is, for the most part, all he does at night: sit in the dark, alone, and listen to his family talk about and without him.