Bodily Oddities

Immersion diuresis

Also called: Needing to pee in water · Needing to pee in the bath or pool

Sitting in water up to your neck makes the kidneys produce extra urine. The main cause is water pressure, not cold. Body-warm water works as well as cool water, and in one study the warmest water had the smallest effect.

Internal organs Heart and blood Reflex Debunked myth Try it

How common

A normal body response The evidence comes from small lab groups, mostly young men. Nobody has counted how many people notice it.

Does this happen to you?

How it works

A common idea is that swimming makes you need to pee because the water is cold. Cold is not needed23. The main cause is water pressure.

Water presses on the body4. It pushes blood out of the legs and belly into the chest1. Stretch sensors in the heart and lungs read this as too much fluid1. The hormones that hold water in fall, including vasopressin, also called antidiuretic hormone or ADH1. The kidneys then pass more water and salt1.

In one study, eight men sat in water up to the neck for five hours. Urine flow rose about sixfold at 32 °C and 34.5 °C, but only threefold at 36 °C2. The warmest water gave the smallest effect. The title of a 1990 letter says the effect does not depend on temperature from 25 °C to 35 °C6.

In another study, only quite cold water added to the effect. One hour in water raised urine output by 107% at 32 °C and by 89% at 20 °C, which was not a significant difference3. At 14 °C it rose by 163%3.

Warm water may also start urination in another way. In a 1993 study, 16 of 30 healthy volunteers urinated without trying in a 40 °C hip bath5. The author proposed a “thermo-sphincter reflex” that relaxes the bladder outlet5. This rests on one small study by one author.

How common it is

Nobody has counted it. The evidence comes from lab studies of small groups of young men23. They report average rises in urine output, not how many people feel an urge.

Try it

Use body-warm water, never hot water. Stay awake, and get out if you feel dizzy.

  1. Empty your bladder and note the time.
  2. Sit in a bath at about 34.5 °C, with the water up to your neck2.
  3. Stay for about an hour3. Note when you first need to urinate, compared with a normal hour on dry land.

Is it a problem?

No. It is a normal kidney response to water pressure1.

Mention it to a doctor if urinating hurts, if there is blood in the urine, or if you also go much more often than usual on dry land. Those have other causes.

History and culture

Lab studies of the effect go back to at least 19627.

A sleepover prank puts a sleeping person’s hand in warm water to make them wet the bed. No study supports it. The TV show MythBusters tried it on three people, and nobody wet the bed8. Three people are too few to settle it. But the studies above put the body in water up to the neck, not one hand12.

Sources and further material

  1. PubMed Water immersion and the kidney: implications for volume regulation Epstein M, 1984 Undersea Biomed Res 11(2):113-21. Review. Head-out immersion moves blood into the chest, lowers renin, aldosterone and vasopressin, and raises urine and salt output.
  2. doi.org Effect of water temperature on diuresis-natriuresis: AVP, ANP, and urodilatin during immersion in men Nakamitsu S, Sagawa S, Miki K, et al., 1994 J Appl Physiol 77(4):1919-25. Eight men, five hours, head-out. Urine flow rose sixfold at 32 and 34.5 °C, threefold at 36 °C.
  3. doi.org Human physiological responses to immersion into water of different temperatures Šrámek P, Šimečková M, Janský L, Šavlíková J, Vybíral S, 2000 Eur J Appl Physiol 81(5):436-42. One hour of head-out immersion at 32, 20 and 14 °C in young men.
  4. Wikipedia Diuresis Has a section on immersion diuresis. Its claim about the hand prank rests on a source that Wikipedia itself flags as weak.
  5. doi.org Role of warm water bath in inducing micturition in postoperative urinary retention after anorectal operations Shafik A, 1993 Urol Int 50(4):213-7. 30 healthy volunteers and 21 patients in warm hip baths. Proposes a "thermo-sphincter reflex".
More sources for this oddity (3)
  1. PubMed Immersion diuresis occurs independently of water temperatures in the range 25 degrees-35 degrees C Knight DR, Horvath SM, 1990 Undersea Biomed Res 17(3):255-6. A letter commenting on a 1989 paper. It has no abstract. Only its title supports the claim.
  2. doi.org Diuresis associated with prolonged water immersion Graveline DE, Jackson MM, 1962 J Appl Physiol 17:519-24. An early lab study of the effect.
  3. mentalfloss.com Does the 'Hand in Warm Water' Trick Really Make People Pee in Their Sleep? Matt Soniak, Mental Floss, 2023 Describes the MythBusters test of the prank. Three people, no wet beds.

Published .

Missing something your body does? Tell me about it.

Organs Reflex

Cold diuresis

Being cold makes you need to urinate. The body pulls blood away from the skin into the core, the heart reads this as too much blood, and the kidneys are told to dump water.

Everyone
Heart Reflex

Diving reflex

Cold water on the face makes the heart slow down, sends blood to the brain, and holds the breath. Every mammal studied so far has it. You can trigger it with a bowl of cold water.

Everyone
Skin Involuntary

Pruney fingers

Fingertips and toes wrinkle after several minutes in water. This is not just the skin soaking up water. It is the nervous system shrinking the fingertip on purpose, and the wrinkles may improve grip when wet.

Nearly everyone with intact nerves
Nerves Involuntary

Pee shivers

A brief involuntary shudder during or just after urinating. Many people seem to get it, men more often than women. It is widely known and almost completely unstudied.

Nobody has counted it