Star Trek

 I've been watching the original Star Trek and there's something they never
addressed on there. Lets say Captain Kirk works an eight hour shift as captain,
eight hours off and eight to sleep. This means that there has to be two other
Captain's of The Enterprise. There's also two other First Officers.
  Who are the other two Captains and First Officer?
  What happens if Captain Kirk is relaxing in his room and the Klingons attack?
  Does Captain Kirk run to the bridge and the Number 1 Captain, or does he stay
there watching movies and let the other Captain take over?
Kirk goes to the bridge while the Klingons are shooting at the ship and the
other Captain says, "Your fifteen minutes late, where the hell were you. Take
over, we're getting clobbered. I want to get a drink, it's been a hard day."