Way too serious.
The tone of this movie was so heavy. It kinda stripped all of the fun out of it because it only seemed to partly work as a thriller. With the slow pacing it kinda sucked the life out of me.
There was literally only one light-hearted moment, which was the, "Any chance you can punch me in the face?" line. That drew 3 or 4 chuckles out of the 13 suffocating souls in the movie theater.