Idiom: To do something in one's sleep
Sean: Did you finish the math homework yet?
Terry: Are you kidding? It was so easy I could've done it in my sleep.
Sean: Oh, come on! It wasn't that easy!
Crissy: I heard your really good at making lasagna. Do you think you could teach me?
Sarah: Sure, I make it every week for my husband because he loves it. In fact, I've made it so many times and I could do it in my sleep.
Crissy: Sounds great!
Meaning: "to do something in one's sleep" is used when something becomes so easy to do that you don't even have to really think about it or concentrate very much any more.