The former Lizzie McGuire star welcomed a baby boy with husband Mike Comrie in March.
"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," Duff told People.
"Then it hits you - the fatigue," she added. "So many people were like, 'Take naps when your baby naps'. 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."
Duff recently admitted that she has been forced to compromise her approach to certain tasks since becoming a mother.
The actress has said that she feels pressure to lose weight after giving birth last month.