Definition of one could do something in one's sleep in English:

one could do something in one's sleep

phrase

informal
  • One could do something with no effort or conscious thought.

    ‘it's a cinch. I could do it in my sleep’
    ‘she knew the music perfectly, could sing it in her sleep’
    • ‘You could recite stopping distances and road signs in your sleep.’
    • ‘The ability to execute simple skills at the highest level at pace was absent, whereas the All Blacks seemed to be able to do it in their sleep.’
    • ‘The instructions in issue 10 are spare, intended for Craftsman-level builders who frame walls in their sleep.’
    • ‘Do this until you feel so comfy with every note you could sing the tune in your sleep.’
    • ‘Listening to the arguments for pornography is like listening to the refrain of a song one can sing in one's sleep.’
    • ‘By now, they can probably give them in their sleep.’
    • ‘They could do it in their sleep, which some of them looked very near.’
    • ‘But when you've been at this (the record business) as long as me, you can think in your sleep.’
    • ‘By this stage, you could build a movie in your sleep.’
    • ‘The company's line is so practised that they can recite it in their sleep: more jobs, more choice - and what's more, shoppers want it.’