Diving reflex

How it works

The reflex starts when your face, and especially your nose, gets cold and wet while you hold your breath¹. The trigeminal nerve senses this and reports to the brainstem. The brainstem replies through the vagus nerve, which slows the heart¹².

Three things happen at once³:

• The heart rate drops. In adults it usually falls by 10% to 25%¹.
• Blood vessels in the arms and legs tighten. Blood is pushed toward the heart and brain, where oxygen matters most³.
• Breathing stops³.

The spleen also contracts and releases stored red blood cells, which carry extra oxygen¹.

The reflex is stronger in infants than in adults³. In the first year of life, a wet face is enough to trigger it fully. Adults also need to hold their breath³. It does not make babies safe in water.

It belongs to a family of trigeminal reflexes that slow the heart¹. Another is the oculocardiac reflex, set off by pressure on the eyeball⁶.

Doctors use cold water on the face to stop a fast heart rhythm called supraventricular tachycardia³. It raises vagus nerve activity, which slows electrical signals through the heart. It does not always work⁵.

How common it is

The reflex is found in all air-breathing vertebrates studied so far, including humans, so there is no population survey¹. Its strength varies. The Haenyeo divers of South Korea show a pronounced drop in heart rate, and the Sama-Bajau of Southeast Asia have enlarged spleens¹.

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 stronger¹.
2. Take your pulse for 15 seconds and multiply by 4.
3. Take a normal breath. Do not breathe in and out fast first. That is dangerous⁴.
4. Hold your breath and put your face into the water, forehead and nose included, for as long as is comfortable. Lift it out if you need air⁵. Stop if you feel dizzy.
5. Lift your face out and take your pulse again at once.

Expect a drop of about 10% to 25%¹.

Is it a problem?

No. The diving reflex is a normal protective response³. In hospital use, complications are rare. When they happen, the heart usually slows more than intended or pauses briefly⁵. The reflex can also cause extra or irregular heartbeats¹. Putting your face under water carries a risk of breathing water in⁵.

See a doctor before trying it if you have a heart condition. Never rely on it alone to fix a racing heart.