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Rafael Nadal is a man of routine, so about 45 minutes before facing Roger Federer in the 2008 Wimbledon final, he stepped into the shower and turned the water on as usual.

He wanted it freezing cold.

The cold shower ritual, which Nadal kept before every match, transported him to a different plane. He felt his power surge and his resilience grow. When he emerged, he was, in his words, “a different man.”

When Nadal chronicled his routine in his 2011 autobiography, he wrote of feeling “activated” and “in flow.” He said he entered “a state of alert concentration in which the body moves by pure instinct, like a fish in a current.”

“Nothing else exists but the battle ahead,” he wrote.

As a creature of habit and superstition, Nadal was not so different from many of the world’s best athletes. Still, as a believer in the power of a cold shower, he may have been ahead of his time.

These days, the sports world is inundated with cold-water immersion — morning cold plunges, postgame ice baths, the nascent trend of dipping your face into a bucket of cold water, also known as the mammalian dive reflex.

Nadal’s routine helped produce a staggering 14 French Open titles and 22 Grand Slam titles. As this year’s French Open approached, my editor asked if I wanted to try Nadal’s routine — to implement a cold shower into my weekly routine of 5Ks, pickup basketball runs and tennis matches — a little DIY journalism to understand the effect.

My first stop after agreeing brought me to Sylvain Laborde, a professor and researcher at German Sport University Cologne who has spent the last two decades studying self-regulation techniques for athletes. Laborde’s primary area of emphasis is heart rate variability, or how well the body can adapt to stress.

As Laborde explains, an athlete’s heart rate variability is a marker of their vagus nerve activity, which functions as a major component of the parasympathetic nervous system, and acts as a communication line from the brain to the heart.

“The take-home message,” Laborde told me, “is that when you find a way to activate this nerve, you will be better able to regulate yourself.”

When I first emailed Laborde about the effect of a cold shower, he was excited. (“I love questions about Nadal routines!” he emailed back.)

He also said something that surprised me: While a cold shower could have clear benefits for an athlete, why stop there?

Our conversation led me on a cold-water journey, from embracing freezing cold showers like Nadal, to surviving ice baths favored by many athletes, to sticking my head in a bucket of cold water.

Trust me, Laborde seemed to be saying.

Step 1: The benefits of a freezing shower

Six days after I emailed Laborde, I decided to try my first cold shower.

It was a humid spring morning in New York, the temperature was close to 80 degrees before 9 a.m., and I had played pickup basketball the night before. My hamstrings felt balled up; my back was stiff. Worse, I was still feeling the jet lag from a trip to the West Coast.

Still, I wanted to attempt a short jog before sitting down to work, and a cold shower seemed like the perfect antidote. So I turned the water as cold as it would go and attempted my best imitation of Nadal.

What happened next may not come as a surprise to connoisseurs of the cold plunge. First came the cold shock response. Then there is an increase in heart rate and a process called vasoconstriction, in which the blood vessels near your skin tighten. I felt an undeniable rush of adrenaline and energy, or what Laborde calls sympathetic activation.

Five minutes later, I wanted to start my run.

The feeling, however, was fleeting. A mile in, my muscles still felt sore again. I wasn’t running any faster than usual. The cold shock response was gone.

According to Laborde, that wasn’t unusual. A decade ago, a group of researchers in the Netherlands studied the impact of cold showers on health and attendance at work. In the study, more than 3,000 healthy adults were randomly assigned to four groups and asked to do one of the following regimens for 30 days:

1. Warm shower only

2. Warm shower plus 30 seconds of cold

3. Warm shower plus 60 seconds cold

4. Warm shower plus 90 seconds cold

The researchers came away with two interesting findings. The participants who regularly finished their showers with cold water were less likely to miss work due to illness, but still just as likely to get sick.

The second part: The length of time under cold water seemed to have almost no effect; a 30-second cold shower was nearly as effective as the 90 seconds.

The results suggested two things: A cold shower could potentially mask symptoms of illness and make someone feel better and more alert for a short time, but the effect was not likely to last long. It wouldn’t actually affect your health.

Put in an athletic context, a cold shower might give someone like Nadal a burst of energy — a sympathetic activation — in the moments before a match. However, once on the court, they would soon be on their own.

Of course, there are other reasons a pregame shower routine might be beneficial. Research has documented the positive psychological effects of pre-performance routines, whether it be visualization, cue words or a shower. And if a cold shower provides marginal gains in the opening games of a match, as Nadal believed, well, that’s something.

Step 2: Experimenting with an ice bath

Laborde grew up in France, where he dreamed of becoming a professional cyclist and competing in the Tour de France. Still, his interest in self-regulation techniques began with one of the most infamous moments in the country’s sporting history.

In 2006, when Laborde was completing his first master’s in sports science, he watched Zinedine Zidane, one of France’s all-time great players, lose his cool and head-butt Italy’s Marco Materazzi in the final minutes of a World Cup final, drawing a red card.

For a young sports fan, the moment was shocking. It inspired Laborde to want to better understand how the body’s nervous system worked. There had to be a way to stop someone from head-butting their opponent in the heat of battle.

“I started to research the different methods and techniques that could help in not doing that again, basically,” he said.

The tools included simple techniques such as deep breathing, and trendier methods such as the cold plunge — or ice bath — which Laborde told me would have an even greater impact than a shower.

You’ve probably seen athletes touting their cold plunges. LeBron James. Novak Djokovic. A pair of baseball teammates at spring training. Sometimes it’s for recovery; other times just a morning jolt. Just two minutes in 50-degree water might help you sleep better or feel stronger.

The science, Laborde said, is compelling.

After a morning basketball workout, I walked home, drank a cup of coffee and filled my bathtub with cold water (and dumped in a few ice cubes for good measure).

First off, an important disclaimer: For those with heart issues, an ice bath can cause serious complications. For those in good health, there is still uncertainty about how the body will respond. Laborde recommends doing an ice bath with someone nearby.

As for the effects, the ice water creates the same cold shock response as the shower. However, the full-body immersion causes hydrostatic pressure — the natural weight of the water pushing against your body — that should, after the initial cold shock, stimulate the vagus nerve.

It’s why some people describe a feeling of calm after the initial shock wears off.

I never quite felt the calm. Well, maybe a little bit afterward, but I also felt tense. The water was freezing.

Ninety seconds in the tub felt like 10, and I was ready to get out.

Step 3: Trying the dive reflex

Fortunately, there is a method much easier — and even quicker — than an ice bath. Laborde does it every day after lunch.

Sitting at his office desk, he takes a bowl of cold water, plunges his face into the bowl and keeps his face submerged for as long as he can hold his breath.

While this exercise may feel like something reserved for TikTok influencers, Laborde says might be useful for athletes before or after training, or maybe even at halftime.

The process triggers the diving reflex, which sends competing signals to the brain, or something called an autonomic conflict.

First, the act of putting your face in water causes bradycardia — the heart rate slows, your body conserves oxygen and blood flows to the heart and brain. The parasympathetic nervous system is activated, putting you in rest-and-digest mode.

At the same time, the cold water on your face activates the trigeminal nerve, which activates your fight-or-flight response.

“You are extremely aware of your surroundings and you can adjust to the situation extremely rapidly,” he said. “So this makes you very adaptive.”

In other words, it’s the perfect state for an elite athlete: vigilant but calm.

“You can decrease impulsivity,” Laborde said. “But you have faster reaction time.”

Laborde’s argument is based on science and research, but there is also the benefit of belief and routine. That is what Nadal understood about his pre-match cold showers. It was a psychological boost as much as anything, a way to start to rev himself up to perform at his peak when the time came.

On the day I wrote the final paragraphs of this story, I filled up a bowl with ice water and tried the dive reflex again. Ten minutes later, I could feel something.

Maybe I was just excited to be done with a story, but I understood what he meant.

I was excited — and relaxed.