answer: Mike tells Mary that all the flying makes him exhausted and it is very hard to get used to the difference between different time zones. Mary suggests that Mike gets some rest.
documents: #Person1#: Hey Mike, good to have you back. You look exhausted.
  #Person2#: Hi, Mary. Yeah. I'm totally beat. I can barely keep my eyes open.
  #Person1#: Was it a rough trip?
  #Person2#: Well, it was actually pretty productive. But all the flying really got to me.
  #Person1#: Oh, jet lag.
  #Person2#: Yep. I flew from Beijing to Boston for a meeting and then got on a plane to go to the trade fair in Frankfurt. Then back to Beijing before catching a train back here to Shanghai.
  #Person1#: Wow, that's a lot of traveling. No wonder you're exhausted.
  #Person2#: The worst thing was adjusting to the time zones. It's so hard to get used to the difference.
  #Person1#: Yeah, it's hard. You know jet lag only hits if you travel east west or west east. You could fly from Germany to Cape Town in South Africa and you wouldn't feel a thing.
  #Person2#: Right, because it's all in the same time zone. Unfortunately for me, all my travel was between different time zones. I've got to say, I'm really suffering. Why is jet lag so nasty?
  #Person1#: I know what you mean. Well, you'll be OK once you get some rest.