Jessica Biel is "constantly dirty" since becoming a parent.

Jessica Biel

Jessica Biel

The 34-year-old actress - who has son Silas, 16 months, with husband Justin Timberlake - admits she has always favoured "casual" clothing but these days she dresses down even more as her little boy's grubby hands always manage to "ruin" her nice garments.

She told People magazine: "I have always been more of a casual dresser in general, but definitely [since] becoming a mom, I just find myself constantly dirty.

"I put on a nice outfit and two seconds later I've got tiny spaghetti-hand marks all over, and I end up ruining things all the time."

And Jessica admitted she splashes out less on her wardrobe these days as she's reluctant to spend money on things she'll only wear for a short amount of time before it gets "ruined".

She said: "It's been this interesting evolution of, 'OK, what can I find that's going to be durable that I can wear and maybe throw a heel on if I need to do a quick transition from day to night and that I'm not gonna spend a bazillion dollars on and then it gets ruined?' "

But when she does want to look glamorous, the 'Total Recall' star makes sure she dresses up when Silas isn't around.

She laughed: "I pick things that I don't wear when my kid's awake or [I'm] anywhere near him because I'm terrified," she admits with a laugh."

However, for Jessica, "practicality" usually comes first when buying new clothes these days.

She said: "It's literally all for practicality and ease of lifestyle."

While Silas influences his mother's fashion choices, his father Justin recently admitted the little boy inspires his songwriting.

Discussing his track 'Can't Stop The Feeling', he said: "I'd be leaving out the whole truth if I didn't say having him shaped what the song actually is. [Having a child] gives you a different insight. It's like 'The Matrix,' man. You open a door and there's this whole other world on the other side of it. And you look back through the door at the other world and you're like, 'What was I doing there?' "