A hyperactive, or over-stimulated, vagus nerve sometimes leads to fainting, according to Mayo Clinic. This condition, vasovagal syncope, occurs because the nervous system is not properly controlling heart rate and blood pressure.
Vasovagal syncope has many possible causes, such as being overheated, seeing blood, having blood drawn, being afraid and straining muscles for a prolonged period, Mayo Clinic explains. When this trigger occurs, heart rate drops and blood vessels in the legs dilate. Blood collects in the legs, and then blood pressure falls. Low blood pressure and reduced heart rate cut back on blood flow to the brain. This combination induces fainting.