The Mammalian Diving Reflex: How Washing Your Face Slows Your Heart Rate 

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Many people don’t think twice about washing their face. You might stumble out of bed in the morning, still somewhat bleary-eyed from your slumber, and head to the sink to splash your face with cold water.

It can feel like an instinct that has become so programmed into your daily morning ritual that you overlook some of the many benefits of washing your face. Did you know that submerging your face in cold water can actually trigger something called the mammalian diving reflex?

Although there are other routes to trigger the divers reflex or dive response, most people do not have ready access to a body of water, and some scientists believe that just the simple action of washing your face in cold water can trigger the mammalian diving reflex.

But what is the mammalian diving reflex? How does washing your face stimulate the mammalian dive response? Are there benefits of the mammalian diving reflex for anxiety, or what about divers reflex for hangovers?

Keep reading to learn about the mammalian diving reflex and how washing your face in cold water may have more benefits than just washing away sleepiness from the night before.

In this guide, we will cover: 

  • What Is the Mammalian Diving Reflex?
  • What Happens During The Mammalian Diving Reflex?
  • Benefits of the Mammalian Diving Response

Let’s get started!

A person swimming in the ocean submerged underwater.

What Is the Mammalian Diving Reflex?

The mammalian diving reflex, also called the mammalian dive response or the mammalian dive reflex is an innate reflex that humans and other mammals have that is activated when we hold our breath or when our face is cooled.

If you think about what happens when you dive into a deep pool of water, both things occur: you hold your breath, and your face gets cooled.

In essence, because humans and other mammals rely on oxygen in the air to breathe, the mammalian dive response is a physiological protective mechanism that allows the body to withstand a lower level of oxygen, as it is impossible to breathe air underwater.

There are different ways to activate your mammalian diving reflex, such as through voluntary breath holding and washing your face in cold water.

What Happens During The Mammalian Diving Reflex?

There are three primary physiological changes that occur during the mammalian divers reflex:

A person splashing water in their face.

#1: Your Heart Rate Slows Down

The dive response triggers the autonomic nervous system to decrease your heart rate. Experts suggest that the human heart rate slows down between 10 to 30% during the mammalian diving response and up to 50% in trained breath holders.

Your heart rate might decrease to the point of bradycardia, which is typically considered a heart rate below 60 bpm.

Your heart rate slows almost immediately upon placing your face in cold water.

The cold water stimulates a cranial nerve (one of twelve pairs of nerves that emerge directly from the brain rather than the spinal cord of other nerves) known as the trigeminal nerve.

The trigeminal facial nerve, which is the fifth cranial nerve, transmits sensory information from the face to the brain. 

There are three primary sensory signals that are transmitted via this nerve: pain, touch, and temperature. Therefore, almost immediately after exposure to cold water on the face, the brain receives a signal that then is innervated by the vagus nerve (the 10th cranial nerve), which, in turn, slows your heart rate.

The bradycardia, or slowing of the heart rate response, occurs more quickly the colder the water temperature is.

Thus, icy water will yield an even faster mammalian diving response reaction.

Water temperatures above 21°C (70°F) do not elicit the mammalian diving reflex or a heart rate response. 

A person diving into a lake.

#2: Your Blood Vessels Constrict

In order to ensure that your vital organs, such as your heart, lungs, brain, and internal organs are able to receive enough oxygen, the blood vessels in your extremities and the surface of your skin constrict to reduce blood flow to these less essential parts of the body.

Reduction in peripheral blood flow, or peripheral vasoconstriction, occurs more gradually in response to breath holding or exposure to cold water on the face.

Reduced blood flow to the limbs is a preventive reflex that is initiated just before the level of oxygen in your body becomes critically low, so just a slight or temporary decrease in blood oxygen levels is usually not sufficient to induce significant peripheral vasoconstriction.

The purpose of decreasing circulation to the limbs is that by shutting blood away from the periphery of the body, it helps conserve oxygenated blood by creating a smaller circulatory system between the lungs, heart, and brain.

This helps ensure that oxygen-sensitive organs are better able to attain enough oxygen to survive during times of a relative oxygen shortage.

A deep sea diver experiencing the Mammalian Diving Reflex.

#3: A Systemic Blood Shift Can Occur

Although not likely to occur with basic breath-holding exercises or washing your face with cold water, during deep dives, one of the changes associated with the mammalian diving response is a blood shift. This refers to changes that occur in blood plasma flow.

Essentially, blood plasma, the liquid portion of blood, the void of the cells, along with water, is allowed to pass through the walls of the blood vessels to perfuse the organs.

In particular, the lungs are filled with blood plasma in order to protect them from the increase in pressure that occurs deep under the water. 

Unlike air, fluids cannot be compressed, so by filling up the lungs with blood plasma, the body is employing a protective mechanism to prevent the lungs from collapsing from the high pressures experienced deep underwater.

The lungs and vital organs that gradually fill up with blood plasma will return to normal when the pressure drops when you resurface.

A person underwater in a pool holding their breath.

Benefits of the Mammalian Diving Response

One of the primary benefits of triggering the mammalian dive response is that it is thought to help increase your carbon dioxide tolerance.

When we exercise, ventilation increases both in terms of breathing rate and tidal volume, or breathing depth.

Although some of the increases in respiration are certainly attributable to the increased demand for oxygen by the heart and working muscles, respiration also increases as a way to offload excess carbon dioxide.

We inhale oxygen and exhale carbon dioxide, so as carbon dioxide builds up, the autonomic nervous system instinctually triggers an increase in respiration rate in order to decrease carbon dioxide levels back to the tolerable range.

Carbon dioxide is a waste product generated through energy metabolism, particularly during the reliance on anaerobic glycolysis during intense exercise.

A person swimming at the bottom of the pool.

In the absence of an adequate amount of oxygen, muscle cells are forced to generate energy through anaerobic glycolysis. This produces the end product of lactate, instead of pyruvate, along with hydrogen ions, which are essentially an acid.

The lactate can get shuttled to the liver to be further broken down for energy, but the acidic hydrogen ions build up in the muscles and blood, decreasing the pH of your blood.

The body begins buffering this acidic buildup in order to try to maintain homeostasis as much as possible.

Carbon dioxide is one of the waste products that is produced through the energy generation and acidic buffering process.

The body is very sensitive to carbon dioxide levels, and even a slight increase in the concentration of carbon dioxide in your body will trigger a resultant increase in respiration rate in order to expel this excess carbon dioxide.

A person splashing water in their face.

This can lead to more forceful and tiring breathing during exercise and a feeling of overall breathlessness.

By improving your carbon dioxide tolerance through voluntary breath holding and cold water face washing that triggers the mammalian diving response, you can help train your body to handle higher levels of carbon dioxide before needing to increase your breathing so dramatically.

This, in turn, may improve your exercise performance and increase your time to exhaustion during vigorous exercise.

But what about the mammalian diving reflex for anxiety? Or divers reflex for hangovers?

Studies have also found that cold water face washing can also decrease stress and anxiety, increase heart rate variability, and may be mildly beneficial for hangovers.

Overall, the mammalian dive response is a survival mechanism. 

A person splashing water in their face.

The decrease in heart rate helps lower oxygen consumption because the heart is a highly consuming organ in terms of oxygen needs.

Slowing down the heart rate means that the oxygen demand from the heart decreases because the heart is working at a lower intensity since it is not contracting as many times per minute. 

Peripheral vasoconstriction and shutting off blood from the limbs help conserve oxygen for the most oxygen-sensitive organs, and the blood shifting helps ensure that the organs, such as the lungs, do not collapse from the increased underwater pressure.

Do you find that you are easily out of breath while running or working out in general? If you want to work on your breathing technique to not feel so out of breath while exercising, read our article How To Breath While Running: 5 Tips To Make Breathing Effortless for more information.

A person, hands on knees, out of breath from running.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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