Claire’s* legs ache with cold as she pushes forwards into the messy grey sea, grey sand stretching behind her, grey sky above. She braces herself against each incoming wave, the wind whipping at her exposed skin. This is the chilling reality of cold water swimming.

Her local beach, Scheveningen, on the western coast of the Netherlands, is a wide expanse of sand running uninterrupted beside the North Sea. It’s March, and the sea temperature is about 6°C. In the water, Claire’s skin temperature drops instantly, and after a few minutes, her muscles start to cool, stiffening like chewing gum.

Her swim is short, and warming up takes hours, but she’s elated to be there.

“We were jumping around, shrieking like schoolgirls,” she says, remembering her first taste of cold water swimming. Claire needed the boost, as three months before, after suffering a personal trauma, she’d sunk into severe depression.

In the UK, where Claire is originally from, researchers are looking into the scientific benefits of cold water swimming for people who are experiencing mental health problems, such as anxiety and depression.

They’re dipping volunteers into troughs of frigid water in labs, and leading groups into the water beside Brighton Pier. And they’re discovering that cold water immersion can prime you, mentally and physically, to better deal with any stress that might come your way.

How swimming in cold water shocks the body

One man who’s leading the research into cold water swimming is Prof Mike Tipton, an environmental physiologist at the University of Portsmouth. An open water swimmer himself, he studies how people react to sudden immersion.

He says that the mood benefits of cold water swimming can be divided into two phases: the initial ‘cold shock’ response, and then adaptation that happens over the longer term.

If you’ve ever taken a wintry dip, you’ll recognise cold water shock. First, you gasp involuntarily, then hyperventilate. Adrenaline courses through your body. Your heart races. You panic. Although you can’t sense it, your blood pressure is skyrocketing, and glucose and fats are being released into your bloodstream, providing an energy source should you need to make a quick escape. This is the classic ‘fight-or-flight’ response.

Cortisol, a stress hormone, is released from your adrenal glands, which maintains this state for minutes to hours, while a surge of beta-endorphin hormones in the brain provides pain relief and gives a sense of euphoria. This explains Claire’s post-swim high, which felt so good that she and her friend made it a ritual. Every Sunday, they rode their bikes to the beach to go for a dip.

In Portsmouth, Tipton puts his volunteers through a formalised version of Claire’s weekly dips to measure how they adapt to cold shock. He sits his volunteers in a hanging chair, lowers them into a trough of water at 12°C, and keeps them there for about five minutes.

Tipton notes that it only takes six immersions to halve the cold water shock response. In other words, your body learns to adapt: your heart and breathing rates only rise half as much, you panic less and you can control your breathing. This adaptation makes you less reactive to the shock of cold water, but it could also make you less reactive to everyday stress.

This is what researchers call ‘cross-adaptation’: adapt to one stressor, and you can partially adapt to others. Tipton’s colleague Dr Heather Massey first demonstrated cross-adaptation in humans by habituating her volunteers to cold water, before putting them in a low-oxygen environment to simulate high altitude. The cold-adapted volunteers had a much smaller stress response to the lack of oxygen than other volunteers who hadn’t been cold-adapted.

Tipton believes that this cross-adaptation could lessen your response to psychological stress, too. Although physical and psychological stress can affect the body in different ways, they also share common elements.

“These stressors are always stimulating the sympathetic nervous system [the part of the nervous system that’s responsible for our ‘fight-or-flight’ response], as well as other systems responsible for cellular tolerance to stress,” says Tipton. “We think that cold water habituation resets those systems to deal with stress better.”

Can cold water swimming help with depression?

While Tipton has been studying cross-adaptation in his lab, Dr Mark Harper, an anaesthetist at Brighton and Sussex University Hospitals, has been looking at the same principle at the Brighton seafront.

He became interested in the potential stress benefits of cold water swimming because anaesthetised patients also go through a large physiological stress during surgery, and minimising that response could help patients to recover.

While he has no current plans to test whether swimming in cold water could help improve surgical outcomes, it is nonetheless an interesting hypothesis.

He’s especially fascinated in one specific component of the stress response. Just as stress causes an adrenaline surge, preparing us to attack or to run, it also kick-starts the immune system in preparation for possible wounding or infection.

This protective response, called inflammation, is healthy when stressful events are rare and isolated, but it can become chronic when people experience stress every day.

What’s more, chronic inflammation has been linked to depression. One study of identical twins found that the twin with higher levels of inflammation was more likely to have developed depression when revisited five years later.

And a recent meta-analysis, pooling 36 studies that covered more than 9,000 patients, found that taking anti-inflammatory medication alongside antidepressants reduced depression symptoms, when compared to taking antidepressants alone.

So if adapting to the stress of cold water can decrease our general stress response and reduce inflammation, it follows that it could potentially reduce our risk of depression too. However, Harper is quick to point out that depression is complex, and inflammation is likely to be one of a number of factors that play a role.

Many open water swimmers talk about how their hobby has benefitted their mental and physical health, and it’s not hard to see why. The exercise itself is good for the body, and spending time by water is associated with increased wellbeing. Swimming with others fosters a sense of community, and completing a challenging task creates a sense of achievement.

Harper recognises the importance of such a broad therapy: he’s planning a study using cold water swimming to improve the mental wellbeing of NHS workers who have struggled through the coronavirus pandemic.

Back in the Netherlands, Claire swum regularly over the course of a year, underwent talking therapy, and took part in mindfulness training. When she was well enough to return to work, she took up a post as the mental health curriculum leader at the school where she teaches, ensuring that the students grow up knowing how to protect their own mental health.

She cites cold water swimming as a really important part of her own recovery. When she was struggling the most, it gave her social contact and didn’t require her to be at her best. “We didn’t need to talk,” she says, “we could just go out and shout in the sea.”

• * name has been changed
• This article first appeared in issue 351 of BBC Science Focus

How to cold water swim safely

• If you have a history of heart disease or suffer from asthma, talk to your GP before you swim.
• Swim with other people. 
• People go cold water swimming anywhere that’s deep enough for a dip: the sea, rivers, lakes, quarries, lidos. Choose a safe site, and make sure you know how to exit the water before you get in.
• Wear a wetsuit if it makes you feel more comfortable, but the colder you feel, the better the adaptation – build up to just wearing a swimming costume.
• In winter, add neoprene boots and gloves to keep your hands and feet warm, as well as a woollen hat over a silicone swimming cap.
• Ease yourself in slowly. At first, it’ll be hard to control your breathing, so it’s best to get used to the cold where you can touch the bottom or hold onto a ladder.
• After 15 to 20 minutes in the water, you might begin to stiffen up. If you start to feel like you can’t maintain a swimming stroke, get out of the water.
• Once you’re out, your body will continue to cool for about half an hour. Put on warm clothes, have a hot drink, and wait until you’ve warmed up before driving home.