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The mammalian diving reflex optimizes respiration which allows mammals to stay underwater for a long time. Both a conscious and an unconscious person can survive longer without oxygen under water than in a comparable situation on dry land.

Beyond the effect of the reflex when submerged in water, the reflex is used consciously in “splashing cold water on one’s face.” This is used both to relax when upset and to wake up when drowsy.

(via lauren-ity)