Day-to-Night Star Transformations

Gwen Stefani

For the Sweet Escape singer, dressing up is a round-the-clock activity. Her off-hours looks may be chosen for comfort (she is, after all, a new mom), but Stefani always has on her signature face-sculpted brows, shimmering skin and red, red lips-and her bright platinum locks swept into a stylized updo. "Gwen mixes anarchy and old-school glamour," Christian Dior designer John Galliano has said, and that is never clearer than when she's on the red carpet. After dark, Stefani draws inspiration from the early days of the silver screen, donning corseted dresses by Vivienne Westwood or Christian Dior.

-Betony Toht

Sienna Miller

"I'm really not good at dressing up and being glamorous," Miller has said. She sure has us fooled. The 25-year-old star of Factory Girl looks as comfortable in gowns by Marchesa and Chanel as she does in the streetwear that has made her a fashion icon. Her secret seems to be keeping the makeup minimal (she favors golden skin and '60s-style winged eyeliner) and her hair styled with a bit of edge. When she attended the recent Golden Globe Awards, hairstylist Oscar Blandi gave her a "modern and updated version" of a coronet of Heidi braids. A cutting-edge classic? That's so Sienna.

Jessica Biel

Biel has become the woman to watch on the red carpet. "She's starting to see herself more as somebody who could be a fashion girl," said Cristina Ehrlich, who styles the actress along with Estee Stanley. "We've found a look that really works for her: sleek, fitted dresses and things that show off her body in a very unobvious way." During the day, the sporty star is seen in workout wear or jeans, but at night she goes Old Hollywood in sleek vintage bandage dresses from designers like Hervae Laeger. And with that sculpted shape-remember the workout wear?-all eyes are sure to be on her.

Jennifer Aniston

"She really knows how to choose clothes that suit her," Donatella Versace has said of Aniston. "Sleek, simple, romantic. She follows her own instincts." Her instincts have led her to a daytime look of impeccable separates from designers like Calvin Klein. After dark, she walks the red carpet in tailored dresses and subtle hair and makeup. As her friend jewelry designer Jennifer Meyer has said, "She never tries to be something she's not."

Cameron Diaz

Diaz has got the look. With her tall, thin model frame, she can wear just about anything-and, luckily for us style stalkers, she often does. When not bikini-clad on a surfboard, she throws together lean daytime layers with boho flair. After dark, she'll bump it up a notch, but always keeps her cool. Lately she's been donning sleek minis that show off her mile-long legs. Day or night, she keeps her dos on the simple side, letting her newly dark mane speak for itself. Why'd this California girl go from blond to brown? Well, Cameron's just got to be Cameron. "I changed my hair color," she said recently, "because in my mind's eye, I feel like a brunette."

Angelina Jolie

For Angelina Jolie, fashion is as simple as black and white … and gray. During the day, she wears jeans paired with cashmere sweaters and shawls in her favorite shades for a comfortable yet pulled-together look. "The move is toward sophisticated elegance," her hairstylist Ted Gibson has said of the star's style evolution. At night, she shows off her raw beauty in effortless gowns-in, you guessed it, black, white or gray-worn with eye makeup aa la Brigitte Bardot. "She's polished yet unafraid to take risks," says Stefano Gabbana of Dolce amp Gabbana. "She knows what it means to be a woman."

Scarlett Johansson

Johansson has a strong sense of occasion. She keeps her daywear simple ("I'm pretty casual-I wear sneakers and flats."). But she goes all out after dark, because, as she says, "I like to look like a lady." The sultry star shows off her curves in vintage-style gowns nipped at the waist, but is careful to show restraint. "It's great to celebrate sexuality," she has said. "Not to overemphasize it but touch on it." She tries out retro-flavored bouffant dos for an overall look inspired by the silver screen. "For the red carpet, I pull pictures of things I've seen in movies," Johansson has said. "I don't flip through magazines to see what I should be wearing."

Katie Holmes

Holmes keeps her chic around the clock. During the day-whether shopping at Barneys or chasing her baby-she dons fitted jeans and slim-cut jackets or sweaters, along with practical flats from Hogan or Marc Jacobs. After dark, she goes for high glamour in gowns from good friend (and designer of her wedding dress) Giorgio Armani. "He is amazing, and I let him do his work," she told MSN recently, referring to the couturier's custom designs. Armani has been putting the brunet beauty in sleek beaded numbers in pale colors that set off her natural coloring. And forget the flats-the statuesque (she's 5'9") star is happy to leg it on the red carpet in heels.