could (do something) in (one's) sleep(redirected from could in their 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.