could (do something) in (one's) sleep

could (do something) in (one's) sleep

Is 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 could do it in my sleep! You should ask Johnny for help—he could do this level of math in his sleep.
See also: could, sleep