Hypervigilance is a hyperarousal state in which you are contantly on guard to detect any danger in the environment. Hypervigilance is a common symptom of PTSD (post traumatic stress disorder). PTSD occurs from a life-threatening event that induces severe anxiety symptoms, characterized by nightmares and flashbacks about the trauma, avoidance of any reminders of the trauma, and hypervigilance for any remaining or future traumas. When you have hypervigilance, you constantly scan your environment for anything that resembles the trauma or threat. Hypervigilance is part of a hyperarousal state that is also characterized by irritability, sleep difficulties, poor concentration, and feeling on-edge and tense.
We can also look at the neurobiology of hypervigilance and hyperarousal. The amygdala, a part of the brain which has a central role in the experience of anxiety, has reciprocal connections with the locus coeruleus, which is the main site in the brain that produces norepinephrine. With PTSD, you are frequently exposed to the memories of the trauma that are stored in another part of the brain called the hippocampus. The hippocampus then activates the amygdala, which then forces the amygdala to activate other regions of the brain to produce a fear response. The amygdala then triggers the locus coeruleus, and this sets off the fight or flight response, which are the physical symptoms of the fear response. The fight or flight response is the peripheral manifestations of anxiety and fear.
In addition, the activated locus coeruleus triggers the amygdala, and this triggers central symptoms of anxiety and fear, characterized by hyperarousal (hypervigilance), nightmares, and flashbacks.
So hypervigilance is a hyperarousal state, caused by exposure to traumatic events, and is seen in PTSD. The locus coeruleus sends excessive norepinephrine to the amygdala, and this induces hypervigilance and hyperarousal. You can reduce hypervigilance by taking a beta blocker (propranolol) or alpha blocker (prazosin), which blocks the adrenergic receptors in the amygdala (and also adrenergic receptors in the body, such as the heart).