I googled a while back, I found three shifts... Night, Day and Early.
It's possible early shift and the day shift is the same thing, but Janeway said that they couldn't run the ship with less than a hundred people, which one would assume means two shifts, so with 150 crew it's more than possible they had three Spartan shifts on Voyager.
This is how I would imagine Night Shift works.
There's a planet 5 hours at maximum speed, the day shift isn't on duty for another 7 hours, so lets go a little slower than maximum speed, get to the planet in 9 hours so that the day shift has got it's shit together by the time they have to make any decisions.
Harry was given command of the ship in Futures End during a hairy tense battle situation.
There was an episode were Harry told Tom to get some command experience and sign up for some night shift duty as acting Captain... But lets be clear about this, the first thing the acting captain of Night shift is supposed to do the second anything goes pear shaped is to wake up Chakotay or Tuvok. If things get really bad, those two try to wake up Janeway.