In Denmark, the cold bath is not just an influencer protocol. In 2025, 6.9% of people aged 15 and above winter-bathed at least monthly, up 2.6 percentage points from 2020, according to SDU's Danmark i Bevægelse survey. It is a social ritual as much as a wellness intervention.
Cold exposure does large, measurable things to the body. That is not the same as knowing what it does for you over time. We have cold tubs on the terrace and we like them. That is exactly why the distinction matters.
Measured
Cold changes the body quickly. A measured stress response is not proof of a lasting health benefit.
Narrow finding
Cold water immediately after resistance training may reduce muscle growth or strength adaptation. That comes from specific protocols, mostly in young men.
Not established
There is no proven general-health dose for immunity, mood, fat loss, longevity, or general wellbeing.
A response is not the same as a benefit
Cold water can trigger an involuntary gasp, rapid breathing, and abrupt changes in heart rate and blood pressure. In a laboratory study of ten young men, an hour immersed to the neck in 14°C water raised metabolic rate by 350%. That is a big response. It is not a three-minute plunge, and it is not evidence of a lasting health benefit.
The viral 250% dopamine figure is real for that exact protocol. The PubMed abstract reports plasma dopamine rising 250% and noradrenaline 530% during one hour in 14°C water in ten young men. It still does not transfer to a short plunge, or establish a predictable mood benefit.
Cold can also activate brown fat. A scan of 24 men found activity during mild 16°C air cooling. It was cool air, not water, and it measured neither weight loss nor fat loss.
What heat and cold together have actually shown
After hard exercise, cold or contrast bathing may reduce perceived muscle soreness compared with passive rest. That is sports-recovery evidence, not evidence that sauna and cold water improve general health. The older 18-trial review found low study quality and no advantage over other recovery methods.
The direct health evidence for contrast therapy is thinner. A 2026 systematic review covered 21 studies and 853 people. It found little consistent evidence for resting heart rate, blood pressure, vascular health, glucose, or lipids, and most studies were small. Heat and cold are real stressors. Combining them does not yet make an established health treatment.
If you lift weights, cold water immediately after training is the place where the evidence points away from cold. A 2024 review pooled eight studies with 116 participants, mostly young men using about 10°C water for 10 to 20 minutes within minutes of their last set. Muscle growth was greater in the groups that skipped it. That does not tell us about cold later in the day, on a rest day, or between sauna rounds.
Nobody has established a dose for general health
Exercise-recovery studies test narrow protocols for narrow outcomes. Nobody has established a dose for general health or wellbeing.
The large cold-shower trial did not give people an entirely cold shower. For 30 days, 3,018 Dutch adults took their normal warm shower and ended it with 30, 60, or 90 seconds of cold. They reported 29% less sickness absence, but there was no significant difference in days they felt ill or between the three cold durations. Regular physical activity was also associated with lower sickness absence in the data. It was not an exercise experiment.
A 2025 systematic review of 11 randomised trials ranged from 7°C to 15°C and from 30 seconds to two hours, then called for research into the right protocol. More time is not a demonstrated upgrade. There is no prize for staying longer.
A claim is not a measurement
Open each claim to see what a study recorded, and what it still cannot establish.
Cold showers improve immunity
- Recorded
- Self-reported sickness absence.
- Did not significantly change
- Days people felt ill.
- Cannot conclude
- Improved immune function.
More cold gives more benefit
- Recorded
- 30, 60, or 90 seconds at the end of a warm shower.
- Finding
- No significant difference between duration groups.
- Cannot conclude
- That any duration is optimal.
11 minutes per week is scientifically proven
- Recorded
- The habits of eight experienced male winter swimmers who also used sauna.
- Not tested
- Randomised weekly doses.
- Cannot conclude
- A minimum or optimal dose.
| Topic | Population and conditions | Measured | Does not answer |
|---|---|---|---|
| Cold showers | 3,018 adults. Their normal warm shower ended with 30, 60, or 90 seconds of cold for 30 days. | Self-reported sickness absence and illness days. | Immune protection or a best duration. |
| General health and wellbeing | 11 randomised trials, 3,177 participants. Protocols ranged from 7°C to 15°C and 30 seconds to 2 hours. | Stress, sleep, mood, inflammation, immune markers, and wellbeing. | One protocol for general health. |
| Muscle growth | 8 interventions, 116 people, mainly young men. Usually 10°C for 10 to 20 minutes immediately after lifting. | Muscle growth over several weeks. | Cold hours later, on rest days, or after sauna. |
| Contrast therapy | 21 studies, 853 people, with varied heat and cooling protocols. | Cardiometabolic and vascular outcomes. | A reliable health protocol for Heat Harmony’s exact format. |
What the studies do not show
Cold exposure has not been shown to provide a clinical immune benefit. The 2025 review found no significant effect on immune function, while some inflammatory markers rose immediately after immersion. That does not mean no biological signal has ever been observed. A small 2024 trial found changes in immune markers after daily cold showers, but did not measure infections, illness days, or absence.
Mood evidence is thinner than it sounds. One study of 33 people measured self-reported mood immediately after five minutes at about 20°C, without a control group. The 2025 review found no significant pooled mood effect.
The often-quoted 11-minute weekly figure was an observation, not a prescription. Researchers observed the habits of eight experienced male winter swimmers who also used sauna in a 2021 Copenhagen study. They did not randomise people to 5, 11, or 20 minutes, and they did not test a general-health outcome.
Who was actually studied?
The 2025 review counted 3,177 participants. The headline looks broad until you see how that total is built.
10 studies: Men only
1 large study: Women and men, 3,018 participants, cold-shower study
The Finnish sauna result does not change this. A study of 2,315 middle-aged Finnish men found that frequent sauna use was associated with lower cardiovascular and all-cause mortality over two decades. It was observational, studied Finnish dry sauna rather than cold water, and cannot show that sauna caused the difference.
A safety boundary, not a clearance tool
Cold is optional. There is no safe time that applies to everyone. After the sauna, cool briefly in the air, enter slowly and upright, and keep your head and face out of the water. Do not hold your breath. Follow the facilitator's instruction and go straight out if you feel dizzy, unwell, or cannot settle your breathing.
Do not use the tub if you have had alcohol or drugs, or if you feel ill, dizzy, or unwell. Speak with your clinician or midwife before choosing cold exposure if you are pregnant, have heart or vascular disease, high blood pressure, epilepsy, a history of fainting, or take medication that can affect pulse, blood pressure, balance, consciousness, or temperature regulation.
Pregnancy evidence is especially sparse. The 2025 scoping review and its correction identified six studies and eight reports. Its recommendations were mostly expert opinion, and the authors concluded that the evidence is insufficient for accurate safety advice. This is general information, not medical advice or personal clearance.
How we assessed the evidence
We prioritised systematic reviews, randomised trials, and primary physiology research that directly measured cold-water exposure. We distinguish cold water, cold showers, cool air, contrast bathing, and Finnish sauna research because they are not interchangeable. We do not treat influencer claims, vendor guidance, or observational sauna findings as proof of health effects from cold plunges.
- Cain et al., 2025 systematic review
11 randomised trials and 3,177 participants. It covers health and wellbeing outcomes across very different cold protocols, and does not establish one dose.
- Buijze et al., 2016 cold-shower trial
3,018 adults finished normal warm showers with 30, 60, or 90 seconds of cold. It measured self-reported absence and illness days, without a best-duration finding.
- de Haan et al., 2026 contrast-therapy review
21 studies and 853 people. Most studies were small, and the review found little consistent evidence for resting heart rate, blood pressure, vascular health, glucose, or lipids.
- Shawe et al., 2025 pregnancy scoping review and correction
Six studies and eight reports after correction. Most recommendations are expert opinion, and the evidence is insufficient for accurate safety advice.
Heat Harmony information
The order and minutes in our own tubs belong to Saunagus and the cold plunge. Would you rather keep the heat on its own? Relaxed Gus is three gentle rounds without any expectation of a tub afterwards. Signature Gus builds and settles again. Times are in the schedule, current prices on the pricing page, and practical house rules in our FAQ.