Beth waits in silence for the explanation, and it doesn't come. He says his piece, tries to bargain it, and leaves it at that. There isn't really a good explanation, she's betting, because there isn't a way to say You need a rest and I don't without sounding like a jerk. And Wash isn't the kind of guy who likes to be a jerk, as far as she can tell.

"I don't need a break," she tells him. It's not sulky, just a fact. She's done harder things than this, with more dangerous people watching her work. This isn't something she needs a rest from. (It is, actually--but in her mind, it seems simple enough. If she's tired, she needs to push through it.) "But I'll take one if you will."

It seems like the easiest way to get him off her back. He clearly doesn't want to stop his work any more than she does.