Okay. So that's not an entirely unfamiliar concept. People get tired at night, generally. It's maybe not something that he's spent a lot of time studying, but it's a sensible concept.

And he has to suppose that if one were to be somehow transported from one time zone to another, that could cause problems, but he's really not sure how it might do that.

Watson has been running a finger over his moustache thoughtfully, but now he leans forward. "César," he says, in a serious tone of voice, "how long have you been awake?"