IIRC from when I did my night rating, there are three different night time definitions:
The 30 mins after sunset is used to define when you start logging the time as night flying, but I'm sure the above definitions are relevent in some way as well, but without my books to hand I can't say what.
One point is that at northern latitudes in summer, you may not actually get the latter two (and if you go north of the arctic circle, you won't get any of them at midsummer!)