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Cold Water Immersion - Cold Shock, After Drop and Benefits

An article discussing cold water shock, the effect of cold water on the body and the benefits of cold water swimming

Cold Water Immersion

Article by Adam Cowell, Jan 2024


In this article, we are going to talk about cold water immersion. Specifically, intentional cold water immersion, because unintentional cold water immersion, although sharing many of the same responses and natural reflexes, is another topic about life safety and what to do in an emergency.

Over time, our bodies will adapt to the cold water and, in some ways, become less susceptible to the initial shock and response. Over time and with adequate training, one can prolong exposure to cold water and embrace the long-term benefits. However, with as much training as possible, you still can't change the laws of thermodynamics, and your biological response to the cold could cause injury or even death, so I'd like to preface this video with a warning that this is not medical advice, nor am I a biologist - I am a swimmer, and I will refer you to the experts as we go through. Before you enter cold water, ensure you have the means to get out (i.e. someone to help you or the appropriate exit strategy), as your muscles will not function as they usually do, and you will lose some sensation and strength in your extremities.

But first, let's run through the initial effects of cold water on the average human body (and what I mean by that is someone who has not trained in cold water immersion).

Cold Shock

As you immerse yourself in cold water, the water contact on the skin sends a signal to the temperature sensor in the brain (hypothalamus), alerting it to the fact that the body is experiencing cold. The cold water on the skin triggers the cold gasp torso reflex, where the body ingests a sharp, large volume of air into the lungs. This reflex can be a problem if you are in deep water as you may inhale water into the lungs, potentially leading to the person aspirating water and drowning. Avoid this by assertively lowering yourself into the cold water but not too quickly. This is easily done in a cold tub; it may be slightly more challenging in deep water. Never jump into cold water to 'get it over with'.

Hyperventilation: your breathing rate will increase dramatically as your body reacts to the cold water, sometimes increasing as much as ten times the average rate. An excessive breathing rate can lead to exhaustion and reduced performance if you aim to exercise in cold water. Your response to the cold water can be trained, and over time, you can reduce and even eliminate this effect by adapting your body's natural response to cold water immersion, triggering brown fat activation and thermogenesis. I will go on to explain adaptation.

Vasoconstriction: the reduction in size of your blood vessels, forcing your heart to work harder. This process happens as a thermoregulatory response from the hypothalamus to keep warm blood away from your extremities and closer to your vital organs. The narrowing of the blood vessels in the extremities will lead to increased blood pressure and an increased heart rate - putting demand on the heart and can increase the likelihood of cardiac arrest even in the young, fit and healthy.

The cold stressor will affect your body at a cellular level, triggering your sympathetic nervous system and producing a surge of noradrenaline, epinephrine and dopamine neurotransmitters (catecholamines). You may have heard of the fight or flight response; in the context of cold water, it produces the effects on the body, as mentioned earlier.

How cold is cold?

Firstly, there is a difference between being hot and being cold before getting into cold water (i.e. sauna to cold, outdoor swimming to cold). I will refer you to Dr Susanna Søberg's research on this and why the Søberg principle coined by Dr Andrew Huberman is beneficial. The principle discusses the differences between your body being warm or cold before entering cold water.

Water below 15 degrees will generally feel cold to your body; however, to an outdoor swimmer, that is almost warm water. The International Ice Swimming Association (IISA) [7] and the International Winter Swimming Association (IWSA) [8] have similar competition guidelines. These guidelines differentiate between swimming in ice water at −2 to +2°C, freezing water at +2.1 to 5°C, and cold water at +5.1 to +9°C [6]. Water as low as 5 or 6 degrees can be tolerated by the body for an extended period, with some people training to be in almost freezing water, but to answer this question - water that feels uncomfortably cold will produce the 'cold shock reaction' activating your sympathetic nervous system and the release of catecholamines. Ice baths, or cold plunge pools as they are known, are a great way of producing this effect, but a cold shower, dip in a cold lido, pool, the sea or lake, and even spending time outside in the winter months in a t-shirt will also produce this cold shock response. Cold water immersion is the most effective way to expose your cold receptors.

What are the benefits of cold water immersion?

Just like with exercise, cold water immersion, when carried out safely, can be highly beneficial to long-term health. I will again point you to Dr Søberg's research [1] and conversation with Dr Huberman [2], where they talk about this in great detail, and the European Journal of Physiology in the study of catecholamines after cold water exposure [10]. To summarise, cold water exposure releases catecholamines - the neurotransmitters dopamine, norepinephrine, and epinephrine (and more), for a long time after exiting the cold water. Dopamine levels in the body can increase by 2.5x and last for 4-5 hours or longer after exposure. Dopamine can have a positive impact on mental health, mindset and wellbeing.

Felling more comfortable in cold weather: With the adaptation to the cold and the activation of brown fat in the body, you will feel less vulnerable to colder temperatures both in deliberate exposure and generally as you go about your day.

Metabolism increases as the mitochondria in the brown fat increase and become more efficient at heating you; this has a caloric cost, thereby increasing your metabolic rate essentially at rest and can help with weight loss and fat burning.

Gibas Dorna's 2016 study [9] of middle-aged men and women throughout a winter swimming season showed that exposure to cold water during exercise led to an overall reduction in blood pressure, resting heart rate and improvement in insulin sensitivity. Repeated cold exposure (along with exercise in cold water) has many benefits but, in short, can reduce the risk of lifestyle diseases (type 2 diabetes and certain cancers) and reduce blood pressure and inflammation within the body, aiding in recovery from injury, surgery and exercise.


Cold shock will increase your breathing rate, causing you to hyperventilate. With training and over time, you can reduce and even eliminate this effect of cold water, removing the initial gasp reflex to the cold water and increasing your ability to withstand more prolonged exposure to the cold.

This training should be over several months, and time spent in the water should increase gradually to allow the body to adapt gradually and activate our brown fat.

Brown fat is a healthy fat tissue that is our body's first response to maintaining body temperature, a thermoregulator, and muscle tissue is the second layer that begins to shiver, which increases our body temperature; this is called thermogenesis. When the cold receptors in the skin activate the cold shock response via the hypothalamus in the brain, neurotransmitters, specifically noradrenaline, activate the brown fat. The cold receptors in our skin can also activate the brown fat directly, which humans have evolved to regulate body temperature and keep us in homeostasis (the correct body temperature).

Shivering is good! During the cold exposure or the after-drop (explained later), your muscles will shiver; this generates heat and should be viewed as a form of training, whereby your body learns to recover its temperature efficiently by working and activating the brown fat to regain core body temperature.

The capillaries in your skin will also improve their ability to constrict (which is your body's way of preparing you for the subsequent cold immersion); therefore, you will see a difference in your resistance to the cold water.

Over time, this process will become faster and more efficient in your body, help you endure more extended periods of sustained cold water exposure, and warm your body up quicker after the cold water immersion. Again, to mention the laws of thermodynamics, no amount of training will overcome the cold water's ability to reduce your core temperature, and one should be aware that hypothermia can set in very quickly.

After-drop ('the drop')

After-drop is where the body's core temperature continues to lower after exiting the cold water. This phenomenon occurs when the body is immersed in cold water; the blood vessels constrict to keep warm blood around the vital organs and reduce heat loss via the extremities. After you get out and your skin begins to warm, the blood vessels dilate (open up), and blood travels around the body more freely towards the extremities. The warm blood travels to the cold tissue and transfers heat to warm the tissue to restore it to a regular temperature. The blood in the extremities also experiences heat loss to the air via the increased surface area of your skin; this cools the blood in the vessels and travels back through the core, further lowering the core temperature.

The movement of the cold blood into your core and the resulting reduction in the core body temperature will cause you to shiver: a fast contraction of your muscles to create heat in your body (thermogenesis), which also activates brown fat and allows your body to begin to regulate itself, warming up gradually. Shiver is good!


Hypothermia (classified as a core temperature below 35 degrees C) is displayed with symptoms such as shivering, confusion, slurred speech, blue lips and muscle fatigue.

The issue with hypothermia and cold water can be that the person suffering too low a body temperature who has intentionally submersed themselves in cold water may not be aware they are at a dangerously low body temperature. The situation may be relatively safe, i.e. a cold water bath, or incredibly unsafe, such as a large body of water or the sea.

When swimming in a cold lake or sea or cold deep water, the ability to exit that water could be compromised due to fatigue and confusion, leading to a potentially deadly situation, so caution must be taken when attempting any cold water swim or winter/spring month outdoor swim.


1. Dr Susana Søberg - Cell Reports Metabolism (2021)

2. Dr Andrew Huberman & Dr Susana Søberg- Podcast: How to Use Cold & Heat Exposure to Improve Your Health -

3. RLNI - Know the Risks -

4. Wim Hoff -

5. NHS - Hypothermia -

6. Esperland D, de Weerd L, Mercer JB. Health effects of voluntary exposure to cold water - a continuing subject of debate. Int J Circumpolar Health. 2022 -


8. International Winter Swimming Association Water Classification. 2019.

9. Gibas-Dorna M, Chęcińska Z, Korek E, Kupsz J, Sowińska A, Krauss H. Cold Water Swimming Beneficially Modulates Insulin Sensitivity in Middle-Aged Individuals. J Aging Phys Act. 2016 Oct;24(4):547-554. doi: 10.1123/japa.2015-0222. Epub 2016 Aug 24. PMID: 26966319.

10. Srámek P, Simecková M, Janský L, Savlíková J, Vybíral S. Human physiological responses to immersion into water of different temperatures. Eur J Appl Physiol. 2000 March

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