Diving reflex

How it works

The reflex starts with cold on the face, especially the forehead, the area around the eyes, and the nose. The trigeminal nerve senses the cold and reports to the brainstem. The brainstem replies through the vagus nerve, which slows the heart¹².

Three things happen at once¹:

• The heart rate drops. In most people it falls by 10% to 30%¹. In trained divers it can fall by more than 50%¹.
• Blood vessels in the arms and legs tighten. Blood is pushed toward the heart and brain, where oxygen matters most.
• Breathing stops. The throat closes so that water cannot enter the airway.

The spleen also contracts and pushes stored red blood cells into circulation, which adds oxygen-carrying capacity¹. Together these changes stretch the time a mammal can survive under water. Seals and whales rely on the reflex. Humans keep a weaker version¹.

The reflex is strongest in babies under six months, which is why infants can be seen holding their breath and slowing their heart when their face goes under water¹. It does not make babies safe in water. Freedivers train it to extend their dives⁴.

Doctors use it. A fast heart rhythm called supraventricular tachycardia can sometimes be stopped by plunging the face into ice water, because the vagus nerve resets the rhythm³.

Try it

Do not do this if you have a heart condition, and never do it alone in a bath or pool.

1. Sit at a table. Fill a large bowl with cold water. Adding ice makes the effect stronger.
2. Take your pulse for 15 seconds and multiply by 4. Write the number down.
3. Take a normal breath. Do not breathe in and out fast first. That is dangerous.
4. Hold your breath and put your face into the water, forehead and nose included, for 20 to 30 seconds.
5. Lift your face out and take your pulse again at once.

Most people see a clear drop. A cold, wet cloth pressed over the forehead and eyes gives a smaller version of the same effect¹.