Below is an overview of the 2018 film to help you prepare your paper.
The phrase is an "indication that it is night". It serves as a metaphor for the absence of moral oversight or divine protection during the darkness of the night, where "innocence is juxtaposed with increasing guilt". It contrasts the safety of his sleeping daughter ("the angel") with the atrocities he is committing outside.
If you are writing an academic paper, you might consider these thematic angles: When Angels Sleep (2018) - IMDb
Germán, a successful businessman and family man, is driving home late at night for his daughter’s birthday. Exhausted, he falls asleep at the wheel and accidentally hits a teenage girl, Gloria. Her friend, Silvia (played by Ester Expósito), witnesses the accident and, in a state of shock and paranoia, believes Germán is trying to kill them. What follows is a dark, overnight chase through the countryside as Germán’s desperation to protect his reputation leads him to commit increasingly horrific acts.
This title most commonly refers to the directed by Gonzalo Bendala (released internationally as When Angels Sleep ), though it may also be confused with the similarly titled book Donde los ángeles no duermen by María Teresa Colominas.
The film is a psychological thriller that explores the moral decay of an "ordinary man" pushed to extremes by a single accidental event.