Mammalian diving reflex
Answer by Dan Schellenberg:
Instantly calm yourself down via the ‘Mammalian diving reflex’ which is induced when we hold our breath with face submerged in cool water (or ice on forehead). The mammalian diving reflex is an evolutionary trait in all known mammals including humans, although it is less pronounced. The reflex results in a set of complex physiological adaptations we have evolved that puts the body into oxygen saving mode to maximize the time that can be spent under water.
-Immediately upon facial contact with cold water, the human heart rate slows down ten to twenty-five percent.
-Peripheral vasoconstriction sets in. When under high pressure induced by deep diving, capillaries in the extremities start closing off, stopping blood circulation to those areas.
-Instantly calming down an over-active nervous system is helpful for ourselves and others.