When Cameron Diaz appeared on "The Tonight Show With Jay Leno" earlier this month to promote her new movie "The Holiday," she shocked the masses. Wearing a zebra-print dress, the quintessentially blond actress had become an effervescent, dark-haired mystery woman.

"For some reason, in my mind, I'm a brunette," the statuesque beauty told Leno. "But it's kind of fun because I'm a natural blonde, so I still get to use that excuse. People react differently to me. I get hit on by different guys. The darker guys seem to come at you as well, the guys who are more introspective."

Diaz made the bold move back in September, just in time for her boyfriend Justin Timberlake's

FutureSex/LoveSounds record-release party. It was a complete 360 for the "Charlie's Angels" actress, who has been associated with her blond hair since her "There's Something About Mary" days. Diaz's black hair seems to bring out the actress' sky-blue eyes and accentuate the chiseled angles on her face.

But Diaz isn't the only blond star to undergo the refreshing change. Ashley Olsen has opted for a darker shade after many years as a streaky blonde. So have Lindsay Lohan, Hilary Duff and Nicole Richie. (Full disclosure: Duff dyed her hair for a movie she is currently shooting called "Brand Hauser: Stuff Happens," in which she plays a European pop star.) Even Beyoncé has gotten rid of her blond highlights and is now showing off her full, voluptuous dark locks.

[article id="1548654"](See how the ladies' brunet locks compare with their previous blond 'dos in these photos.)[/article]

And let's not forget about Britney, who can't seem to decide between black and blond. It's almost impossible to keep up with Spears' forever-changing locks. In August, when she was pregnant with her second child, she dyed her hair black and wore long extensions to the Teen Choice Awards (see [article id="1538972"]"Britney Introduces K-Fed, Nick Lachey Scores 'Awkward' Award At Teen Choice 2006"[/article]). She dyed it blond again and then, two weeks ago, she dyed it black. Last week, she was spotted at an L.A. Lakers game with her sister, sporting honey-blond highlights. What gives?

Louise O'Connor, owner of prestigious New York salon OC61,

says darkening locks and flipping back and forth "shows how a woman is feeling."

"I think Britney is doing this because she is depressed and confused," she said. "On the other hand, Cameron Diaz looks beautiful, and she has changed her whole look and attitude."

"There was a time when brunettes ruled, like Raquel Welch and Sophia Loren, when brunettes were the sexy hot look," added hairstylist-to-the-stars Edward Tricomi of Warren-Tricomi hair salon. "Now they're bringing sexy back."

Dr. Helen Fisher, an anthropologist at Rutgers University, says blond is outdated and a head full of natural, glowing brown hair looks more youthful.

"The blonde is becoming more and more of the lady who lunches, the kind of woman who does nothing all day except get her hair done," she said. "The blonde used to be regarded as natural, and now everyone knows that it's not. Black hair was always considered sexy and attractive because it signified youth."

Most importantly, with many women being bottle blondes, a brunette stands out -- and men like women who stand out, Fisher believes.

"We're so used to seeing five blondes in a photograph, but when there's a brunette in the middle, that is novel," said the anthropologist, who recently penned a book called "Why We Love: The Nature and Chemistry of Romantic Love." "It may even be a signal of daring, someone who doesn't follow the crowd. Men are very attracted to women who are daring."

Still, as daring as you may be, stylists advise to not go black if you plan on going back to being a blonde.

"You have to be sure about it," O'Connor said. "To go back to blond is very taxing on the hair. It's a definite change. It's going to change your look, how you feel. The attention you get as a blonde is different than the attention you get as a brunette. You are taken more seriously."

Also, if you're going to embrace a darker look, be sure to seek a professional and be wary of drugstore dyes.

"If you're going from blond to dark and your hair has been bleached out and you put a color on top of that, you could end up with a green tone if you put an ash brown in it," O'Connor said. "At a salon, going dark is the cheapest thing to do. You can get a single process and be brown within an hour."

Lastly, O'Connor suggests, try before you dye.

"Make sure it does suit your skin tone," she said. "Maybe get a wig and test that out for a few days before going permanent."

Celebrity hairstylist Oscar Blandi says he's noticed an increase in women coming into his salon and asking for their hair to be darker over the course of the past month.

"A lot of the time it can be a psychological thing," he said. "Britney, Lindsay, they want to find a new star. They are looking for a new identity. They want to start a new image. A lot of the time you find that it looks great, it's a great contrast. Great skin color and blue eyes look great together.

"In Hollywood, people need to reinvent themselves, and a great way to do it is by changing drastically," Blandi continued. "People are getting tired of the Paris Hilton bimbo-blonde look. They want to look intelligent and sexy, and you can be sexy and be a brunette. If you look at the cover of W, nobody is going to tell you Cameron Diaz is not sexy."

Another positive aspect about going brunet is that your can wear your same wardrobe but look completely different.

"If you have a low-cut blouse down to your navel, you might look a little more intelligent with dark hair," Blandi said. "After a while, you get bored of looking at something. When all the magazines are doing blond, blond, it becomes refreshing to see a brunette."