The Portugal star is preparing to return to Old Trafford for the first time since a world record £80million move to Madrid in 2009 when the two clubs meet again on March 5 with the tie evenly poised at 1-1.
And Ronaldo, 28, revealed just how deeply he is affected by defeat, saying: "If we're talking after a big game, a semi-final or final of a major tournament, obviously I do not sleep well after losing.
"And not just the same night either - even for the next two or three. But losing is one of the drawbacks of our profession, even at Real Madrid. It shouldn't happen, but it does."