Hypervigilance is usually associated with PTSD (post traumatic stress disorder). Hypervigilance is is a hyperarousal state where you are constantly scanning your environment for any signs of danger. When you are hypervigilance, you are easily startled, and you are jumpy with any sudden and loud noises.
When you are reminded of the trauma that induced the PTSD, then you become quite anxious and fearful. In an attempt to not endure such fear or anxiety, you become hypervigilant, as you do not want that trauma to occur again, and you constantly scan your environment in order to prepare for any future threats so that you are not caught off-guard again.
In addition, you avoid at all cost any reminders or triggers of the trauma. However, when you avoid the reminders of the trauma, this just maintains your belief that the trauma is still occurring, despite the trauma not being present anymore. Thus, you continue to be hypervigilant and scan your environment for the danger, as you still believe the trauma is occurring when you continue to avoid. This is the paradox of hypervigilance, PTSD, and anxiety in general...avoidance leads to worsening of symptoms.
If you want to calm down hypervigilance, then you need to deal with the PTSD that led to it. The following video explains how PTSD and hypervigilance develops, and what you can do to treat it: