be able to do something in your sleep

be able to (do something) in (one's) sleep

To be able to do something with very little or no difficulty, or complete or accomplish something in a relaxed, carefree, or effortless manner. It took me a little while to get used to this job, but now I'm able to do it in my sleep! You should ask Johnny for help—he's able to do this level of math in his sleep.

be able to do something in your ˈsleep

(informal) be able to do something very easily because you have done it many times before: He’d done the journey so many times that he almost felt as if he could do it in his sleep.