Hypervigilance is normal vigilance stuck at maximum. You notice the exits in a room without meaning to. You startle at a noise nobody else heard. You read the mood of whoever walks in, in three seconds.
It is a learned skill, not a defect. In an environment where detecting a shift in tone early was vital — an unpredictable house, a conflict zone, a relationship that escalated fast — the system calibrated its threshold lower. The problem is that it does not recalibrate on its own when the environment changes.
The cost is exhaustion. Continuous scanning consumes resources, and hypervigilant people frequently describe a tiredness unrelated to how much they have done. Sleep is usually affected first, because falling asleep requires precisely the drop in vigilance they cannot make.