You have stood there with your hand on the tap, not turning it. Here is the number that decides it: thirty seconds, most days. That is the low end of the only range anyone has compared head to head, and in that trial thirty seconds did as much as ninety. The "eleven minutes a week" you have heard is not from a trial at all.
What was actually tested: 30 to 90 seconds, daily
In 2016 a team at the Academic Medical Center in Amsterdam randomly assigned 3,018 adults to finish every shower with cold water for 30, 60 or 90 seconds, or to change nothing, every day for thirty days. It is still the largest test of cold showers anyone has run.
The trial measured seven things. Days off work fell by 29%. Quality-of-life and anxiety scores nudged in the right direction, and the authors themselves called that shift too small to matter clinically. And the one that gets left out: the number of days people were actually ill did not change. They were absent less. They were not shown to be less ill.
Two more things. Nobody was blinded, and people wrote down their own sick days — the authors say plainly that a placebo effect cannot be ruled out. And inside that same trial, plain exercise beat the cold shower: regular physical activity cut sick days by 35% against the shower's 29%, and doing both came to 54%.
The duration finding, read honestly: on sick days off work, 30, 60 and 90 seconds all did the same thing. That is one outcome in one trial. Nobody has compared durations on anything else.
Where "11 minutes a week" comes from, and what it sits next to
From a person, not a study. The figure traces to Andrew Huberman, the Stanford neuroscientist whose podcast popularised it — his reading of the literature, split into two to four sessions. A reasonable convention. Not a finding.
Now hold it next to the dose that has actually changed something in a human body. To switch on brown fat, the calorie-burning tissue everyone talks about, researchers used two hours a day at 17°C for six weeks. Body fat did fall. That is roughly seventy-five times eleven minutes a week. When you read that cold "activates brown fat," that is the dose it took.
What you actually get
The immediate effect is real and nobody disputes its direction. Cold water sets off a fast, large release of norepinephrine, the chemical behind feeling sharply awake. In a study at Charles University in Prague, young men sat in 14°C (57°F) water and their norepinephrine rose 530%, with the rate they burned energy up 350%. That was a full hour up to the neck, not a minute under a shower, and the published summary never says how many men took part — which tells you what weight one number can carry. The same lab ran it again and got a four-fold rise, not five — and when they repeated the immersions three times a week for six weeks, nothing adapted.
Beyond alertness, a 2025 review of eleven randomized trials covering 3,177 people reported better sleep quality and better quality of life, though it did not pool those statistically. Stress fell too, but only when measured twelve hours later, and not at any other time point.
And the one people most expect: mood did not improve. That review found nothing, off a single twenty-person study, and there is essentially no trial evidence on cold showers and depression. If feeling happier is why you were going to start, that is the claim with the least behind it.
What it does not do
Cold does move the machinery. Give ten men ten minutes in 8°C water and nothing else, and the master gene for building mitochondria — the tiny power plants inside every cell — climbs within hours. But when researchers followed that signal for six weeks, the mitochondria themselves had not changed, and neither had the muscle's capacity to use oxygen. A gene switching on is not a body rebuilt.
Nobody has shown that cold water changes how fast a person ages. The protective brain proteins you may have read about come from a mouse study using laboratory cooling, not showers.
Brown fat at shower doses is the same story. Winter swimmers studied at the University of Copenhagen produced more heat in the cold, but their brown fat was not more active — and they paired cold dips with hot sauna, so the credit cannot go to the cold alone. It is a snapshot of two groups of men, not a trial.
The claim that runs backwards
Cold is usually sold as anti-inflammatory. When that 2025 review pooled the trials, inflammation markers were higher right after cold water and higher again an hour later. Read it carefully: they compared each person before and after, not cold against a control group, and in several of those trials people had just exercised — so some of the rise may be the exercise. So the anti-inflammatory story is not supported in the hours right after, and the long-term question is open.
If you lift weights, put the cold on another day
Cold water straight after lifting blunts muscle growth. One trial of sixteen men found size suffered while strength held, but when researchers pooled ten studies, strength was blunted too. Every one of those trials used ten to fifteen minutes of full immersion, not a minute under a shower, and nobody has tested whether a thirty-second shower affects a lifter at all. Keep the cold off your training days.
Who should not do this
Cold water is a real stress on the body, not a wellness accessory.
- Do not do this at all if cold makes you come out in hives. That is cold urticaria, and about a third of people who have it get a whole-body allergic reaction to cold, which in water can kill.
- Talk to your doctor first if you have heart problems, high blood pressure, or any condition affected by sudden blood-pressure changes. The 3,018-person trial deliberately excluded anyone with heart or lung disease, so it tells you nothing about you. In the Prague study, blood pressure and heart rate went up in cold water.
- Never hold your breath or do rapid breathing exercises in or near water. People have drowned this way. Cold water sets off irregular heartbeats even in healthy volunteers — the cold shock speeds the heart while the reflex to cold slows it, and the two fight. Do breathing practice sitting on the floor instead.
- Never cold plunge alone, especially in open water. The first gasp is involuntary — your body does it whether you agree or not.
- Get out if you feel dizzy, if your lips turn blue, or if your shivering stops being something you control.
What to do tomorrow
Tomorrow, do not take a cold shower.
Take your normal one, and at the end, note how many seconds you think you could have stood it. Do that for three days. On the fourth, turn the tap cold for thirty seconds, and then keep it there. Do not climb — the trial found no benefit past thirty, so there is nothing up there to earn.
One thing that decides whether you are doing this at all: "cold" means the coldest your tap runs. In the trial that was Dutch tap water in winter, around 10°C (50°F). If your tap runs lukewarm in August, you are not doing what was tested.
Then let yourself warm up on your own instead of reaching for the hot tap, and keep it off your lifting days. That is the whole protocol, and it is deliberately smaller than the internet's — thirty seconds, most mornings, for how the next hour feels. Not for your mitochondria. Roy takes one most days and says so on his own page, including that he keeps doing it despite everything above. That is a preference, not evidence, and it is worth being clear which is which.
