The expression by night is often used to contrast another person's nighttime things to do to their daytime things to do, especially when the nighttime things to do are strange or unforeseen. You will notice it really frequently in the development X by working day, Y by night. Some examples: https://andresg2s53.bloggadores.com/36876446/night-gangnam-퍼펙트가라오케-karaoke-guide-an-overview