You have probably seen it in movies: When the moon is full, a man turns into a hairy, frightening creature and runs around the city, tearing people to pieces. You may laugh at stories of wolf men, but have you ever wondered where the association of a full moon with crazy goings-on came from?

Such notionshave been around a lot longer than scary movies. The word lunacy, meaning "madness," is derivedfrom Luna, the Latin name for the Roman goddess of the moon. And, a hundred years ago in some European hospitals, fear of violence during full moons led doctors to chain uppatients thought to be lunatics.

It is surprising how many people still believe that the moon can affect human behavior in unusual ways. For example, many medical workers are convinced that the number of emergency cases increases during a full moon. Considerable research has been conducted to establish a link between the cycle of the moon and the biological clocks of humans and other animals, with varying results.

One study in the United States found that people drink less alcohol and eat more during a full moon. Italian researchers found that more babies seem to be born in the one or two days following a full moon. And Britain's largest telecommunications company claims that Internet use increases during the seven days before a full moon.

Other researchers, however, have found serious methodologicalflaws in some of the studies that claim to demonstrate a relationship between a full moon and human behavior. Also, no research has yet found a direct link between the phases of the moon and attacks of depression or mania.

With only weak scientific evidence relating madness to the moon, perhaps the best way to explain the existence of this myth is to call it a relicof the past: Long before electricity, bright moonlight was an invitation for extra nighttime activity. And for some people, even just one night without sleep is enough to induce mania in them—whether or not they have hair on their hands.