Night is the time between the end of evening civil twilight and the beginning of morning civil twilight. As far as the UK is concerned, this is taken to be 30 minutes after sunset to 30 minutes before sunrise. I believe that the official definition of civil twilight is when the sun is between 0 degrees and 6 degrees below the horizon. Nautical twilight is when the sun is between 6 degrees and 12 degrees (or is it 0 and 12?) below the horizon. However, it is 36 years since I sat in my groundschool class learning that, so perhaps someone with more recent knowledge can correct me.
Airclues