Dutch adults who finished their showers cold took fewer sick days off work
The 2016 PLOS ONE trial behind the cold-shower sick-day claim randomly assigned 3,018 working adults in the Netherlands to cold or usual showers. Days spent feeling ill didn't differ significantly between the groups.
The paper
The Effect of Cold Showering on Health and Work: A Randomized Controlled Trial
Buijze GA, Sierevelt IN, van der Heijden BCJM, Dijkgraaf MG, et al.. PLOS ONE, 2016;11(9):e0161749.
See the study at a glance
- Design
- Randomized controlled trial in which everyone knew their group
- People
- 3,018 adults
- Who they were
- Working adults aged 18 to 65 in the Netherlands who didn't already take cold showers and had no severe heart or lung disease. About 56–60% of each group were women, and the average age was about 39.
- What they did
- Three cold groups finished a normal warm shower with 30, 60 or 90 seconds at the coldest tap setting. They did it every day for 30 days in January 2015 and then as they chose for 60 more days. Tap water averaged 10–12°C, and the control group showered as usual.
- How long
- 90 days
These numbers came out of it
−29%
The cold-shower groups reported 29% fewer days off work for sickness than people who showered as usual.
0.89
The cold groups' rate of illness days was 0.89 times the control group's. The plausible range (95% confidence interval) ran from 0.77 to 1.01, so the difference wasn't significant.
0 days
The median person in every group took no sick days off work.
30 s = 90 s
Thirty seconds of cold cut absence as much as 90 seconds did.
Incidence rate ratio with 95% confidence interval
Each row compares the cold-shower groups with usual showering over 90 days
The chart pools the three cold groups. People reported both outcomes themselves in web surveys.
The researchers did this
The Cool Challenge recruited 3,018 working adults in the Netherlands through advertising and social media in December 2014. None of them took cold showers routinely, and the trial excluded people with severe heart or lung disease.Source 1 · Randomized trialThe Effect of Cold Showering on Health and Work: A Randomized Controlled TrialBuijze GA, Sierevelt IN, van der Heijden BCJM, et al.. PLOS ONE, 2016.Read the paper ↗PubMed ↗ The researchers split them at random into four groups. Three ended a normal warm shower with 30, 60 or 90 seconds of cold water, and the fourth kept showering as usual. The cold groups held 798, 727 and 775 people, and the control group 718.
The cold groups showered this way every day through January 2015 and then as they chose for another 60 days. Tap water averaged 10–12°C. People reported everything through web surveys, including the two main outcomes: days off work for sickness and days of illness.Source 1 · Randomized trialThe Effect of Cold Showering on Health and Work: A Randomized Controlled TrialBuijze GA, Sierevelt IN, van der Heijden BCJM, et al.. PLOS ONE, 2016.Read the paper ↗PubMed ↗
Across the cold groups, people took 29% fewer days off sick than the controls (incidence rate ratio 0.71, 95% CI 0.56–0.89). The difference in days of illness wasn't significant (0.89, 0.77–1.01).Source 1 · Randomized trialThe Effect of Cold Showering on Health and Work: A Randomized Controlled TrialBuijze GA, Sierevelt IN, van der Heijden BCJM, et al.. PLOS ONE, 2016.Read the paper ↗PubMed ↗ In every group the median person took no sick days and reported two days of illness. Between 29.4% and 34.0% of people in the cold groups took any sick leave, against 34.8% of controls. Shower length made no difference.
Regular physical activity by itself went with a bigger drop in absence (rate ratio 0.65). The authors' model projected a 54% cut for people who both exercised and took cold showers.Source 1 · Randomized trialThe Effect of Cold Showering on Health and Work: A Randomized Controlled TrialBuijze GA, Sierevelt IN, van der Heijden BCJM, et al.. PLOS ONE, 2016.Read the paper ↗PubMed ↗ About 79% of people completed the 30 days, and 64% kept going afterwards. Mental quality-of-life scores were slightly higher at 30 days, and the edge was gone at every later check.
The main side effect was a cold feeling that lingered in the body for 196 people and in the hands or feet for 257. Three of the 257 had Raynaud's.Source 1 · Randomized trialThe Effect of Cold Showering on Health and Work: A Randomized Controlled TrialBuijze GA, Sierevelt IN, van der Heijden BCJM, et al.. PLOS ONE, 2016.Read the paper ↗PubMed ↗ Four people reported insomnia they linked to evening cold showers. A 2018 correction added the dataset behind one table and changed no numbers.
The study can't tell you these things
- Everyone knew which group they were in, and every outcome came from web surveys people filled in themselves.
- Recruitment drew on Wim Hof's popularity, and the promotional material listed benefits people might expect.
- People who went to work while sick would show up with fewer sick days, and the surveys couldn't pick those cases out.
- The researchers had no way to check whether people stuck to the cold showers, and by 90 days they had lost track of 19.6% of participants.
- The trial was registered only after recruitment had finished. It also ran through an unusually long 2014–15 flu season.
People quote it like this
The 29% figure travels as proof that cold showers keep you from getting sick. It counts days off work that people reported themselves, and the same people didn't report significantly fewer days of illness.Source 1 · Randomized trialThe Effect of Cold Showering on Health and Work: A Randomized Controlled TrialBuijze GA, Sierevelt IN, van der Heijden BCJM, et al.. PLOS ONE, 2016.Read the paper ↗PubMed ↗
We cited these sources
- 01Buijze GA, Sierevelt IN, van der Heijden BCJM, Dijkgraaf MG, Frings-Dresen MHW. The Effect of Cold Showering on Health and Work: A Randomized Controlled Trial. PLOS ONE. 2016;11(9):e0161749. doi:10.1371/journal.pone.0161749 PMID 27631616