When you have hypervigilance, you have increased arousal at all times, which is also known as hyperarousal. So hypervigilance is basically a hyperarousal state, and this is commonly seen in PTSD (post traumatic stress disorder). PTSD develops after you are exposed to a life-threatening trauma, which is characterized by recollections and reminders of the traumatic event (nightmares and flashbacks), and you spend most of your time avoiding any triggers or reminders of the trauma, so that your anxiety will subside. Unfortunately, hypervigilance and hyperarousal is a big problem in PTSD, as you are now on guard and ready to fight any danger that may still be lurking out there that may look like the trauma you had to endure. When you have hypervigilance and hyperarousal, you also have associated symptoms of being irritable, feeling tense or on-edge, sleep problems, and poor concentration.
When you look at the neurobiology of hypervigilance and hyperarousal, the memory of the trauma is stored in a part of the brain called the hippocampus. So any trigger or reminder will bring up the memory of the trauma from the hippocampus, as if it were still occurring. This memory recall in the hippocampus then activates the amygdala, the fear center of the brain, and the amygdala then activates the locus coeruleus, which produces norepinephrine. The locus coerulus then triggers the sympathetic nervous system, which sets off the fight or flight response, which prepares the body to become faster, stronger, and more alert and focused, so that you can either fight or run from the threat. But in PTSD the threat is really from your memories of the trauma, not from some actual threat in the environment.
In addition to the peripheral symptoms of anxiety, the central symptoms of anxiety and fear are triggered by the locus coeruleus activating the amygdala. The central symptoms of anxiety are characterized by hypervigilance, hyperarousal, nightmares, and flashbacks.
So when you have been exposed to a trauma, this can cause hypervigilance and hyperarousal, as seen in PTSD. The amygdala and locus coeruleus have reciprocal connections, where the memory of the trauma from the hippocampus activates the amygdala, and this then activates the locus coeruleus which sets off the peripheral anxiety symptoms in the fight or flight response. And going the other way, the locus coeruleus releases norepinephrine, and this excess norepinephrine then sets off the amygdala, and this induces the central symptoms of anxiety characterized by hypervigilance, hyperarousal, nightmares, and flashbacks.
You can decrease the peripheral and central arousal states by taking a beta blocker such as propranolol to block the effects of the excessive norepinephrine in the amygdala, and to block the epinephrine (adrenaline) receptors in the rest of the body, such as the heart.