What's the deal with the night shift?

What does night even mean in space? Whose night? What planet? What time zone?

Let's say we all agree on an Earth-centric Greenwich mean time. If I'm from Vulcan where the day is 39 hours long, how do I even adapt to a 24 hour day?

Why does nothing important ever happen during the night shift, such that the A-team is never needed? Are the Romulans on the same schedule?

Most importantly though... Why do they turn the lights down during the night shift? So they can sleep on the bridge???