be (like) night and day

To be completely different; to be totally or nearly opposite. Her transformation has really been like night and day. She doesn't even look like the same person since she started the program. A: "Have you seen any improvement in his behavior?" B: "Oh, it's night and day. He's been a model student this month." My brother and I are like night and day. All he cares about is sports, and all I care about is being at the top of my class.