An intriguing piece of a psychological horror story.
To most of us, the horror genre has been classified as one that utilizes ghosts or ghouls to instill fear among the viewers. Horror is not limited to that, however. It can also be seen as a feeling of vulnerability and of an enigma, just as the movie is able to depict it. It did not depict horror by showing it, but letting the viewer experience the vulnerable–a reality that could happen to anyone–made it much more terrifying.
I want to make a point about how the movie is excellently depicted from the lead's perspective. It made a significant allowance for the viewers to see her perspective while having these hallucinations and also showed glimpses of reality when she is abstinence of such.
It indeed is the horror depicted by exposing to us the vulnerability like when the door is open in the dark of night.