Młodzież is purely a collective noun and it refers to young people as a whole.
For abstract youth, see młodość.
For a single young male person, chłopak might be the simplest word to use, but other ones might be better in a given context.
By current Wiktionary definitions: https://en.wiktionary.org/wiki/youth 1 and 2 are młodość, 3 and 4 depend on context, and only 5 is młodzież.
I would add , that "młodzież" can mean a group of young people.
this sentence without context says that young people in general do not sleep at night. But it may mean that a certain group of young people does not sleep at night, and gets in trouble,
or that in the specific group of people ( I have a bus trip in my imagination here) , the "youth" part of them does not sleep.