CDL school will teach you the details, but the broad overview is that there are federal regulations on how many hours you can work, and you must keep a 24/7/365 log of your days whether you're working or not. You need complete logs of the last 8 days, possibly the last two weeks, on any day that you will be driving. CDL class should start you logging right away.

The reasoning is that tired drivers get in accidents, and accidents with 80,000LB trucks are significantly worse than car wrecks.

Dispatch WILL push for you to bend regulations because "THIS CAN'T BE LATE" but seriously, fuck dispatch. They might threaten you with write ups or firing for not getting there on time, but just don't fucking do it.

They won't fire you because if they do, you will win in court. As a corollary, get EVERYTHING in writing, on the qualcomm, peoplenet or whatever message system your company uses. Phone conversations never happened. Don't act on any instructions until it arrives in writing.