[While spending Thanksgiving with the family, someone reported with some diffidence sleeping with a bar of soap, which had dramatically resolved excruciating nocturnal leg cramps. I then discovered quite a literature on this subject.]
Many years ago we received a message from a reader of our syndicated newspaper column about putting soap under the bottom sheet to prevent nighttime leg cramps:
“Under the cover of darkness (so my husband, who is an M.D., wouldn’t see), I slipped a bar of soap under the sheet on my side of the bed. For two nights I continued to have mild leg cramps but by the third night they were gone. I have not had them since.”
Since that early letter we have received hundreds of messages from people all around the world regarding the soap treatment for muscle cramps. Many doctors (and quite a few non-health professionals) are absolutely convinced this is total nonsense. They chalk it up to a placebo effect, largely because they cannot imagine how it could possibly work…
Several years ago, the advice columnist Ann Landers raised a provocative question in her column: does soap at the foot of the bed cure night-time leg cramps? The consensus in the medical community is no: there is no conceivable mechanism by which it could, so any relief derived from this procedure must be due to the placebo effect. In other words, it’s all in the mind.
But if it is indeed a placebo effect, it’s a remarkably strong one. Many people who have suffered for months, if not years, from painful, nocturnal cramps in their legs and feet have found immediate and long-lasting relief just by slipping a thin, innocent bar of soap beneath the sheets. Some even report relief although they were unaware that a bar of soap had been snuck into bed.
Likewise, others whose cramps have mysteriously returned have been nonplussed until they later discover that their bars of soap have fallen from the bed. From the point of view of those who, like us, are trying to solve this mystery, it is fortunate that several websites (including this one) have maintained reports of this unusual treatment and its results…
We decided that although these data were anecdotal, and therefore suspect, we would treat them as if they were scientifically valid, and use them to try to develop an explanation for soap’s seemingly helpful effects. But as soon as we started reading the literature, we realized what an enormous task we had undertaken. The anecdotal literature is vast, and frustratingly contradictory…
It seems to work for many people. Soap in the bed appears to alleviate nocturnal leg cramps.
Relief is immediate and sustained.
Some people report that soap does not work. It appears either to work consistently and well or not at all. There are few cases of partial success.
After a few months, a bar of soap is no longer effective for preventing cramps. It must be replaced. Old soap can be rejuvenated by scoring or shaving it to produce fresh surfaces.
Some subjects have placed the soap between the sheets, and some have placed it under the bottom sheet. Either or both of these methods work.
Some subjects report that direct physical contact between the subject and the soap is desirable, but few claim it is essential.
We hypothesize that it is an as-yet-unidentified molecule present in the soap. This might sound like a stretch, but in fact, this “switching” mechanism is consistent with what we know about the structure of soap.
Soap is a water-swollen gel. When it’s purchased, its moisture content is generally somewhere between 5 to 15 percent. Soap is very porous, and when it’s swollen with water, it permits small, dispersed molecules to pass through it. But when it has aged, its surface dries out, and its surface is a lot less porous, so small molecules can no longer pass through it.
We think that an unknown molecule that diffuses out of the soap gel is responsible for alleviating cramps. As long as the bar is emitting this molecule, the cramps are suppressed. An old bar of soap ceases to emit the molecule as the surface dries out and its resistance to diffusion rises. That’s when the cramps return. The bar can emit again–and once again eliminate cramps–after new, moister, fresher surfaces are exposed by scraping the bar of soap…
After generating this hypothesis, we took a careful look at the list of ingredients on a package of soap, and we found only one possible source of small molecules of a volatile compound: the fragrance. Nearly all soaps contain fragrances or perfumes. Certainly those mentioned in the anecdotal evidence do. And what perfumes are used in soaps? That’s generally top-secret information, held close to the vest by soap manufacturers. But we do know that most soaps contain esters and oils, such as carrot oil and lavender oil (or their synthetic doppelgangers). These compounds are vasodilators. Like the ester nitroglycerin, which is used to alleviate pain caused by angina, they enlarge blood vessels.
The quantity of perfume transmitted to the skin may be small, but it appears to be enough to dilate blood vessels and prevent cramps. We know, of course, that the small, mobile molecules in the fragrances of soap diffuse through its gel to the surface and evaporate. We know because we can smell them. And when you score an old bar of soap, you can smell it all over again, just as strongly as when you first took it from its paper wrapper…
Before it can be accepted, every new hypothesis has to be tested. We would welcome the findings of anyone who might want to test our hypothesis, and we would be eager to see the results. To the research community, which is convinced that ion imbalance is responsible for the initiation of cramps, we say that this suggestion doesn’t challenge that. There is ample room here for research by the academic and medical communities. Our proposed mechanism will surely be under attack within the week. Yet it fits much of the data so well that we suspect that whatever future research results are obtained the final conclusion in this matter will include much of what is written here.