Depends on how your national regulatory authority defines "night." Because it depends based on latitude and season, you can't really go strictly by time unless you have the tables and know your exact position.
In the world of the US FAA, it is defined as
the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac, converted to local time.
Transport Canada published a similar definition, without referring to the American Air Almanac. However, their previous [superseded] definition may be more helpful if you don't have a copy of an Air Almanac handy--
"night" - means the period beginning one half-hour after sunset and ending one half-hour before sunrise and, in respect of any place where the sun does not rise or set daily, the period during which the centre of the sun's disc is more than six degrees below the horizon;
So then, a half-hour after sunset and a half-hour before sunrise generally means when everything is pitch dark. For that reason, note that in Arctic regions during the summer time there is no such thing as night, and in the winter there is no such thing as day.
Does this help somewhat?