RALPH: The word watch derives from the same Germanic word as does our word wake. In Shakespeare’s time, both watch and wake had approximately the same meaning, to be awake, but watch often referred to staying awake in order to tend to a sick person or to keep guard over someone. 
DAVINA: When the doctor says that Lady Macbeth “does the effects of watching”, he means that she’s doing the same things she would do if she were awake. 
RALPH: It’s only in very recent times that the word “watch” has come to mean, not just being awake, but focusing one’s attention on something specific – for example, “to watch a play.”