Many people say, "If you sleep with wet hair, you will catch a cold." But colds are caused by viruses, so wet hair alone cannot give you a cold.

Even so, there may be a small truth. In a British study, people who put their feet in cold water for 20 minutes reported cold symptoms more often a few days later. Being cold may help a virus that is already in your nose.

Sleeping with wet hair is often said to cause colds. Scientifically, this is too simple, because colds are infections caused by viruses. In a 1968 American experiment, putting volunteers who had been given a cold virus in a cold room did not make them more likely to catch a cold.

However, a 2005 study at Cardiff University in the UK found something interesting. Of 180 people, half put their feet in cold water for 20 minutes. Over the next four to five days, 13 of the 90 chilled people reported a cold, compared with 5 of the 90 who were not chilled. The researchers suggest that cooling the body narrows blood vessels in the nose, which may help a hidden virus cause symptoms. However, no virus tests were done.

"Don't go to bed with wet hair or you'll catch a cold" is advice heard around the world, yet it rests on a shaky premise. Colds are viral infections, typically caused by rhinoviruses, and no amount of chilling can produce one without a virus. Early research seemed to settle the matter: in a 1968 study, volunteers inoculated with a rhinovirus were no more likely to become ill when exposed to cold conditions.

In 2005, however, Johnson and Eccles at Cardiff University revisited the question with 180 adults. Half sat with their feet in cold water for twenty minutes, while the others placed their feet in an empty bowl. Within four to five days, 13 of the 90 chilled participants reported cold symptoms, compared with 5 of the 90 controls.

Eccles proposed that chilling triggers a reflex that constricts blood vessels in the nose, weakening local defenses and allowing a virus already present without symptoms to take hold. The findings need cautious interpretation. Symptoms were self-reported, no laboratory tests confirmed infection, and participants obviously knew whether they had been chilled. Wet hair, then, is not a cause of colds, but chilling may act as a trigger for viruses already present.