Hilary Duff says the "fatigue" of having a new baby can take its toll.

The 24-year-old welcomed son Luca Cruz five weeks ago and adores being a new mother but she has revealed her "biggest shock" of having a child.

Hilary told People magazine that the sleepless nights were really hard at first but she's now gotten into a better routine.

"The first week I was like, I'm doing this, I'm good. I can not sleep, I can do this.' I don't know where you get that energy from," Hilary told the publication. "Then it hits you the fatigue."

But she says that she then adhered to great parenting advice she was given during her pregnancy.

"So many people were like, Take naps when your baby naps,'" she recalled. "So I tried and it really helped a lot. I thought when he was napping that was the time I could do all the things I needed to do. It's a fine balance."

And Hilary added that her son is now sleeping a "good five and six hours a night, which is just amazing."

"He's really a good baby," she gushed. "I am so grateful."