Cold plunges, ice baths and early-morning dips are among the most visible wellness habits of the moment, and the claims made for them are often large. This article looks at what the best recent review of randomised trials found, how strong that evidence is, and why the safety advice matters more than the benefits.
What counts as cold-water immersion in the research?
The term covers a wide range of habits: a cold shower, a short bath in chilled water, a dip in a lake or the sea. A 2025 systematic review and meta-analysis in PLOS One, led by Cain and colleagues, set out to pool the randomised trials of cold-water immersion in healthy adults and ask what it does to health and wellbeing.
The review included 11 randomised trials with 3,177 reported participants. Water temperatures ranged from 7°C to 15°C, and exposures ranged from 30 seconds to two hours. Ten of the studies used baths; one used cold showers. That variety is the first thing to understand: “cold water immersion” is not one intervention, so results from a 10-minute bath at 10°C do not automatically apply to a 30-second shower.
How much can 11 trials tell us?
Less than the headlines suggest. One trial, a Dutch study of cold showers taken daily for 30 days, enrolled 3,018 people. That is about 95% of the reported total (3,018 divided by 3,177). The other ten trials were small, with between 7 and 26 participants each, and many were done in athletes or young volunteers after exercise.
The review’s authors listed several limitations:
- Few randomised trials, small samples and limited diversity. Only one study included women.
- Very different protocols for temperature, depth and duration.
- Six of the 11 studies used a single immersion, so long-term effects are unknown.
- Five studies applied cold water after exercise, which can muddy the results.
- Reliance on self-reported outcomes such as sickness absence, and participants who could not be blinded to what they were doing.
- Safety was outside the scope of the review.
The review rated its certainty as “Grade B” for inflammation and stress and “Grade D” (inconsistent or inconclusive) for immunity. In plain terms, this is early-stage evidence, not a settled body of findings.
What did the review find on stress, immunity and inflammation?
The pooled results were mixed.
| Outcome | What the review found |
|---|---|
| Inflammation | Markers rose immediately after immersion and at one hour |
| Stress | No significant change immediately, at one hour, 24 hours or 48 hours; a significant reduction at 12 hours |
| Immunity | No significant change immediately or at one hour |
Two things are worth pausing on. First, the rise in inflammatory markers is a short-term response to a physical shock, and the review does not show it as either good or harmful for long-term health. Second, the stress finding at 12 hours comes from a particular time point in a small number of studies, while the other time points showed nothing. It is an interesting signal, not proof that a cold dip reliably lowers stress.
Taken together, the pooled data do not support claims that cold-water immersion boosts the immune system. The immunity outcomes showed no significant change.
Does a cold shower reduce sick days or improve mood?
Some outcomes could not be pooled and were described one study at a time, which makes them weaker still.
In the large Dutch trial, people assigned to cold showers had roughly 29% lower sickness absence (an incidence rate ratio of 0.71), but there was no difference in the number of days people reported being ill. The absence data were self-reported. A reasonable reading is that people who took cold showers may have been more inclined to go to work while feeling unwell, not that they were less often ill, though the trial cannot settle that.
The same trial found slightly higher mental quality-of-life scores at 30 days, but not at 90 days. A separate small study in 20 men found better sleep scores after cold-water immersion (3.6 against 3.1 on a five-point scale), but that took place after heat training, which complicates interpretation. The same study found no significant difference in mood.
What is cold water shock, and why does it matter?
Cold water shock is the body’s reflex response to sudden immersion in cold water. According to the Environment Agency, quoted on GOV.UK, it can cause you to breathe in water, weaken your muscles and cause immediate heart problems, and cold water can bring on cramp and breathing difficulties very quickly. The same page notes that inland waters can be very cold, no matter how warm the weather.
The British Heart Foundation describes the same reaction in cardiac terms: entering very cold water can cause “cold shock”, with a faster heart rate, higher blood pressure and shortness of breath. Cold water can also lead to hypothermia and raise stress hormone levels, which may increase the risk of abnormal heart rhythms and possibly cardiac arrest.
This is why a controlled cold shower at home and an unplanned plunge into a river are very different things. The first can be stopped in a second; the second can leave a person struggling in water where they cannot stand, cannot easily get out and may be unable to use their arms.
Is cold water swimming safe for your heart?
For most healthy adults, the British Heart Foundation says regular cold water swimming can improve blood pressure and blood fat levels. It adds, however, that swimming makes the heart work harder than activity on land, and colder water adds to that strain.
For people with a heart condition, the charity’s advice is to discuss the risks with a doctor or nurse specialist before swimming at all. It describes water between 26°C and 33°C as the safest range for people with heart conditions and notes that most UK sea water is unlikely to reach that, so a heated pool is likely to be the safer choice. Anyone new to swimming, or returning to it, is also advised to check how to exercise safely in water. Swimming is not necessarily off limits with a heart condition, but cold water swimming may not suit everyone.
If you are curious about the numbers behind heart health, our guide to blood pressure numbers and what they mean explains what the readings show.
How can you lower the risks if you do take the plunge?
The safety sources agree on a short list of habits:
- Do not jump or dive in. Depths vary and hidden hazards, such as unseen currents and reeds, may be present, according to the Environment Agency.
- Keep away from weirs, locks, pipes and sluices, and watch for boat traffic.
- Remember that cold water is a risk whatever the air temperature.
- Swim with a friend or in a group rather than alone, and enter and exit gently so the body can adjust.
- Swim only when you feel well, and wait at least one hour after a meal, as the British Heart Foundation advises.
- Stop if you have chest pain, palpitations, light-headedness or simply do not feel well.
The Environment Agency also points out that rivers such as the Thames that are not designated bathing waters may pose higher risks to human health than designated bathing waters. For Londoners, that is a good reason to choose supervised, designated or organised swimming venues over improvised dips.
When to speak to a GP or NHS 111
Speak to a GP before starting cold-water swimming or cold plunges if you have a heart condition, high blood pressure, or have been told you have an abnormal heart rhythm, or if you are unsure whether it suits you. Stop and seek help if you experience chest pain, palpitations, light-headedness or breathlessness during or after a cold dip. Call 999 in an emergency, such as chest pain that does not ease, collapse, or someone in difficulty in the water. NHS 111 can advise if you are unsure whether symptoms need urgent attention.
A modest claim for a very cold habit
The honest summary is that cold-water immersion has a small, early evidence base: some signals around short-term stress and perceived sleep, no clear effect on immunity, and a clear set of safety concerns, especially for people with heart conditions and for anyone entering open water without preparation. It is a choice, not a health requirement, and it should never replace proven basics such as regular activity, sleep, social connection and medical care.
Habits with firmer foundations are less glamorous. Our explainers on the five ways to wellbeing, how mindfulness stacks up against its claims and building a habit that sticks cover approaches with a longer research record. If you enjoy cold water for its own sake, that is a perfectly good reason to do it, provided it is done carefully and not because of promises about immunity or disease.
This is general information, not a substitute for advice from a doctor or other qualified health professional. If you are unsure whether cold water is safe for you, speak to your GP or call NHS 111.
Sources: PLOS One; PubMed Central; British Heart Foundation; GOV.UK
