The sun is rising over Bronte Beach in Sydney's east, and Norman Swan is "crapping himself".
He watches apprehensively as four inflatable baths are filled with sea water and ice.
Soon he'll be taking the plunge with a group of strangers who are voluntarily going for a soak in cold water — less than 10 degrees Celsius — on a Sunday morning.
Deb Sarah is the founder of this ice bath session. She's been running it since last year.
"I reckon it'll be pretty cold. I got some extra bags [of ice] today just for you," Deb tells Norman.
Deb is one of a growing number of instructors encouraging people to embrace discomfort.
Ice bath sessions have been cropping up all over the country, touting a range of benefits from mental clarity to boosted immunity.
But right now Norman is more concerned about the risks.
"What's the worst thing that's happened to someone who has come to one of your workshops?" he asks.
"They've got tingly fingers and toes," Deb says. "So pretty serious."
"Alright. OK. I think I can cope with that," Norman replies — but he doesn't sound so sure.
Norman spent the days leading up to the ice bath session immersing himself in the evidence for and against cold water immersion.
There are plenty of personal stories about the transformative power of a freezing cold dip. But what does science have to say about it?
We've been sitting in cold water a long time
Claims about the health benefits of water therapy date back centuries.
Hippocrates believed it could "allay weariness". Thomas Jefferson swore by his decades-long habit of soaking his feet in cold water.
Some suggested benefits of cold water immersion include improved mood, boosted immunity, less inflammation and faster muscle recovery.
Mike Tipton, from the University of Portsmouth in the UK, is an expert in cold water immersion. He says the short-term psychological benefits can be attributed to stress hormones.
"That sudden change in skin temperature evokes the fight-or-flight response … so you'll see cortisol, adrenaline, noradrenaline increasing," Professor Tipton says.
"And of course they are alerting, they are awakening, they do make you feel alive, because they are intentionally preparing you to fight or run away."
This response could also explain self-reported immune improvements, with the sudden release of stress hormones jolting the system into action.
But despite being "biologically plausible", the evidence for immune improvement is mixed.
One study on people undertaking repeated immersions found "slight" activation of the immune system. Another concluded cold water had no protective effect against upper-respiratory tract infections in experienced swimmers.
Repeated exposure to the cold — and as a result, stress — might even suppress the immune system. Professor Tipton says more research is needed to settle the debate.
It's a similar story with inflammation.
Cold water immersion has been widely used for post-exercise recovery for decades, and it is known as an effective way to fend off muscle soreness.
However, some studies suggest cold water immersion might actually hinder rather than help recovery in the long-term, slowing down reconstructive processes.
Professor Tipton says the proposed benefits of cold water immersion remain largely anecdotal.
"That's still evidence. But it needs to be followed up with definitive studies."
On the other hand, Professor Tipton says, the risks that come with getting very cold, very fast are without doubt.
Shock response can be dangerous while swimming
Instructor Deb Sarah takes a "safety first" approach. She requires participants to be in good health before stepping into an ice bath.
Cold water immersion is known to trigger a shock response — setting off the instinct to gasp and hyperventilate — that's especially dangerous while swimming.
It can also increase cardiac arrhythmias. Professor Tipton attributes this to competing responses in the body.
"You've got two inputs," he says. "The sympathetic nervous system … and the parasympathetic nervous system."
When the body is submerged up to the shoulders, the sympathetic nervous system is activated and the heart speeds up.
But when the face is splashed or submerged, the parasympathetic nervous system is activated. It wants the body to slow down and conserve oxygen.
Despite the instinct to do otherwise, it's important to continue breathing normally in an ice bath, as holding your breath can further increase the chance of arrhythmias.
Professor Tipton suggests entering the water slowly, limiting immersion to under 10 minutes, and avoiding prolonged breath-holding.
"I can't wait any longer," Norman says, eyeing off the water.
"This is it."
Knowing the risks, Norman approaches the ice bath. The instructor tops it up to ensure he'll get the full experience.
Deb guides him to step into the ice bath, to focus on his breathing, to calm the urge to hyperventilate. Then the two-minute timer begins.
Experience is 'very hard to explain'
A broadcaster to the bitter-cold end, Norman manages to control his breathing long enough to speak.
"Well, I can't pretend that it's not cold," he says. "It is freezing. Not quite as freezing as you'd think though."
The man in the ice bath beside him chimes in, "Are you joking?"
Some participants look calm and in control. Others are clearly fighting the self-protective urge to vacate the ice bath as soon as possible.
"It's very hard to explain to people what water temperature is like," Professor Tipton says.
"It's like trying to explain a toothache. Until you've experienced it, you don't really know."
Professor Tipton came to cold water immersion as a study participant, swimming in experiments before conducting his own.
Asked if he would do it again, knowing what he knows now, Professor Tipton says "ignorance is bliss".
Clambering out of the ice bath, Norman is more relieved than blissful.
Participants are encouraged to go for a dip in the ocean to "warm up" before continuing with their day. One person says it's like getting into a hot tub in comparison.
Norman agrees: "It's significantly warmer than what I just experienced."
While some people are attracted to cold water immersion for the assumed physical benefits, most people at this session are here for the psychological benefits.
Alice has been doing ice baths twice a week for the past four months, and the effect still hasn't worn off.
"It's such a mental thing," she says.
"For someone who's an over-thinker … being in the ice forces me into feeling rather than thinking, because you have to feel, because it's cold!"
Tina jumps in an ice bath two to three times a week.
"I think you do get addicted to it. You get that feeling for the rest of the day … you have lots of energy and you just feel good."
So what's the verdict from Dr Swan?
"Most people you talk to are actually coming for the psychological benefits — that this is a challenge, you go through the challenge, and you feel better for it," he says.
"And I suppose I do too."
With an important caveat: Make sure you're fit and healthy before taking on the challenge, and don't try it without an instructor.
