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I don't know where you are, but if you're in Europe the rules were changed in December, last year.
Night is now defined as from the end of civil twilight after dusk to the beginning of civil twilight at dawn.
These points happen when the centre of the sun's disk is 6° below the horizon.
This is obviously very difficult to judge, and will vary greatly with time of year and latitude, so I have written a small Android app that keeps track of it for you (shameless plug alert):
https://play.google.com/store/apps/details?id=st.crosscheck.niteflitelite
It calculates the beginning and end of night for your location.
I hope it is useful to you.