There are at least three definitions of night time as it pertains to the Federal Aviation Regulations in the United States.
1) The standard definition of "night" is the period between evening civil twilight and morning civil twilight.[1] AirNav is a good source for sunset and sunrise and evening civil twilight and morning civil twilight. An aircraft must be equipped with lights during this time.[2]
2) The period between sunset and sunrise when aircraft position lights must be on.[3]
3) The period between 1 hour after sunset and 1 hour before sunrise which applies to airman currency requirements.[4]
To log night flight training and required landings, one uses the standard definition of night.[1][5]