Be able to do something in your sleep
Definition 1 of 1
Pro Tip 1/3Expert Level
Use this phrase to express a high level of skill or expertise in an activity. 
She can solve complex math problems in her sleep—she's a real expert.
Pro Tip 2/3Colloquial Use
It's informal slang, suitable for casual conversations rather than formal or professional settings. 
Don't worry about the test; you've studied so much, you could do it in your sleep.
Pro Tip 3/3Hyperbole
This phrase is an exaggeration, not to be taken literally—it's just a way of emphasizing proficiency. 
I've made this recipe so many times I could do it in my sleep.