Is it true that placing someone's hands in warm water can cause them to wet the bed?

Stories of summer camp and sleepover pranks are as abundant (and often exaggerated) as tall tales. From short-sheeted beds to the infamous shaving cream and feather combo, these antics are legendary. But how credible is the ultimate warning against being the first to fall asleep: the claim that submerging someone's hand in warm water will make them wet the bed? Should we trust the countless anecdotal accounts, or turn to scientific research, which barely addresses this phenomenon?

The team behind "MythBusters" took on the challenge, spurred by widespread anecdotal reports. In episode 136, "Mini Myth Mayhem," they put the age-old trick to the test on hosts Adam and Jamie, as well as a crew member named Matt. Using moisture sensors to detect accidents and sleep monitors to confirm deep sleep, they conducted the experiment. Despite their efforts, none of the three participants experienced bed-wetting when their hands were submerged in warm water.

However, several flaws in the experimental design cast doubt on their findings. The small sample size and the fact that participants were aware of being observed and the experiment's purpose skewed the results. Jamie never actually fell asleep, Adam was frequently disturbed by his sleep apnea, and Matt's wetness alarm was triggered by spilled water. Additionally, Adam's hand positioning prevented Jamie from properly submerging it. [source: MythBusters]

Efforts to experimentally answer this question have not been particularly successful. Is there a theoretical foundation that supports the possibility of this prank? While immersing the lower body or using warm water bottles has been proven to alleviate post-surgical urinary retention, a wet hand is vastly different from a soaked bed. Alternatively, the power of suggestion might offer some explanation; individuals with shy bladders often use the sound or visualization of running water to stimulate urination.

In the following section, we will delve into both immersion and suggestion, and explore the related question: How does sleep factor into this equation?

Go With the Flow

If the warm water hand prank is effective, how does it bypass the natural mechanisms we develop during childhood to avoid bed-wetting (see sidebar)?

The bladder, a muscular sac, stores urine by relaxing and releases it by contracting. Urine exits through the urethra, a tube that is controlled by two sphincter muscles: the internal urinary sphincter at the bladder neck and the external urinary sphincter further down the urethra. These muscles help regulate the retention and release of urine.

From a medical perspective, the key to addressing postoperative urinary retention is to relax the sphincter muscles. Research indicates that immersing in a warm bath or applying a hot water bottle to the pelvic area can achieve this, likely due to a temperature-induced reflex in the sphincters [sources: Elsamra and Ellsworth; Kulaçoğlu et al.; Shafik].

"Heat and warm water are widely recognized for their relaxing effects," explains Valre Welch, a pediatric nurse practitioner and former president of the Society of Urologic Nurses and Associates.

"They promote blood circulation and provide significant comfort following genital surgery."

However, would this reflex apply if only a hand or a few fingers are submerged, rather than the affected area? The answer is a tentative yes, provided the person is in a very deep sleep or influenced by the power of suggestion.

Studies confirm that the sound or imagery of water can trigger a sense of urinary urgency [source: MUCMN]. This is logical, as the brain and nervous system regulate the micturition reflex—the process of bladder wall contraction and urethral sphincter relaxation in response to bladder pressure.

As the bladder fills and pressure increases, nerve signals from the bladder to the spinal cord intensify. These signals are relayed to the brain's pons and cerebrum, creating the conscious urge to urinate. If urination isn't feasible, the brain responds by signaling the bladder to suppress the reflex. When the time is right, the brain activates the micturition reflex, leading to urination [source: McKinley and O'Loughlin].

During sleep, a full bladder acts like an internal alarm, urging us to wake up and relieve ourselves. However, if someone is overly intoxicated, exhausted, or under the influence of relaxation from a warm water prank, they might sleep through this signal.

"It genuinely happens," Welch states. "It’s a result of deep relaxation."