Victims are haunted by a demonic "old hag" entity that appears during sleep paralysis.
The antagonist can only attack those who believe in its existence—a concept that drives much of the investigative plot. Viewer's "Guide" for the Movie Dead Awake
Depending on what you're looking for, there are several other titles with this name: Dead Awake (2016) - Horror Film Review Victims are haunted by a demonic "old hag"