"I am sick at heart."

Francisco was standing guard all night in the cold winter, leaving him physically drained and freezing. He dreams of the idea of sleep, being able to take rest and warm himself before another shift.

 Why is Francisco sick at heart? The play never answers this, although it is raised as a topic that the audience might care about before being immediately ignored.

And just to make things extra confusing, it’s also possible that the actor playing Francisco was the same one playing Hamlet in which case, there would also be a foreshadowing of the heart-sickness that is obviously central to the character of Hamlet.