The 47-year-old actress discussed motherhood on Wednesday's episode of The Tonight Show starring Jimmy Fallon after welcoming Raddix, her first child, with her husband, singer and musician Benji Madden.
"It's been heaven, Jimmy," Diaz told host Jimmy Fallon. "You get to have every moment, and it goes so fast."
"Literally every single day, there's just leaps and bounds and these things that happen. She's not the same baby that she was yesterday," she said. "It's so gratifying to actually get to see that growth and to be a part of it, and to help just let her be her."
Diaz said seeing her daughter grow is "just amazing."
"It's the best thing that ever happened to both Benj and I," she said of parenthood. "We're just so happy."
Diaz said she keeps "Baby Shark," "Elmo's Song" and Sesame Street on rotation for Raddix, even though she previously vowed to "never play that type of music" for her child.
"But you watch their response and they're so taken by it," she said of her daughter. "It's like she changes. She's like, 'Oh! My people!'"
Diaz and Madden married in January 2015 and announced Raddix's birth in January of this year.
"We are so happy, blessed and grateful to begin this new decade by announcing the birth of our daughter, Raddix Madden," Diaz said on Instagram.
"She has instantly captured our hearts and completed our family," she added.
Diaz said on Instagram Live in April that being a mom is the "best, best, best part" of her life.
Diaz is known for such films as There's Something About Mary, Gangs of New York and Charlie's Angels. She last appeared in a 2014 remake of Annie, and discussed her break from acting in the September issue of InStyle.