"the good night" is not a line from the poem, so I am assuming that it is a metaphorical question. In the poem, itself, he juxtaposes the idealized versions of war and the realities of war. The title, Dulce et Decorum Est is only an abbreviated version of a line written by Horace which was: It is sweet and right to die for one's country. He's showing how it is anything BUT sweet, and only half right.
