If you've ever thought, "She's acting like a lunatic!" you may be onto something. Take a closer look at that word-it derives from "luna," which is Latin for "the moon." And for centuries, people have linked the moon's phases and the positions of the sun and stars with crazy behaviors or events. But is there any truth to these superstitions that we hear about in horoscopes?

The Moon and Insomnia

Before the advent of modern gas and electric lighting (about 200 years ago), the full moon was bright enough to allow people to meet up and work outside after dark-things they couldn't have done on darker nights, shows a UCLA study. That late-night activity would have disrupted people's sleep cycles, leading to insomnia. And lots of research has shown insomnia can trigger higher rates of manic behavior or seizures among people suffering from bipolar disorder or epilepsy, explains Charles Raison, M.D., coauthor of the study.

The Sun and Stars

Research has linked the presence or absence of sunlight in your life to all kinds of significant behavioral factors-but not in the way your psychic tells you. For one, sunlight helps your body produce vitamin D, which research from the Boston University Medical Center shows can lower rates of depression. Rays also helps regulate your hunger and sleep cycles, finds a study from Northwestern. And that's just the tip of the sunlight-mood-behavior iceberg.

But when it comes to the position or alignment of various astral or planetary bodies, the scientific evidence resembles a black hole. One study in the journal Nature (from 1985) found no links between birth signs and character traits. Other older studies turned up similar non-connections. In fact, you have to go back several decades to even find researchers who have looked into the subject of astrology long enough to write a paper debunking it. "There's no scientific evidence-zero-that planets or stars affect human behavior," Raison assures. Most astrological charts or calendars are premised on an old, faulty world views.

The Power of Belief

But if you're a believer, you could see some ripple effects. One study from Ohio University found that people who believed in horoscopes or other aspects of astrology were significantly more likely than skeptics to agree with descriptive statements about themselves attributed to astrology (even though the researchers had made the statements up).

"In science, we call this placebo effect," says Raison. Just as swallowing something your doctor tells you is a pain pill can help you feel better (even if it's just a sugar pill), believing in astrology could affect your outlook and actions, he says. "We look for things or signs that confirm what we already believe. And people who deeply believe in astrology will over-recognize things that confirm their belief."

There's not any harm in that, at least if your interest is casual, Raison adds. "It's like reading fortune cookies. The vast number of people who do it aren't going to make a real or serious decision based on their horoscope." But if you're relying on astrology to help you choose your next job (or boyfriend), you might as well be flipping a coin, he says.