Eh, he's a little off-key.
Brazilian patient Anthony Kulkamp Dias was having a tumor removed from his brain and needed some way to keep awake. Also to keep from screaming, "THEY'RE IN MY BRAIN! PEOPLE ARE TOUCHING MY BRAIN RIGHT NOW! HELP! HELP! HELP!" which is what I imagine myself doing under the circumstances.
That's right. You have to stay awake during brain surgery so they can figure out right away if they've lopped off anything important. Usually, patients are asked to keep up a stream of chatter or to recite the alphabet. Dias wanted to play the guitar instead. He does a pretty good rendition of "Yesterday" and it makes an eerie soundtrack to having your head cut open. Compared to your brain surgery day, most normal days must seem pretty trouble-free.
Keep believing in yesterday, Mr. Dias, we are all rooting for your tomorrow!