Reese Witherspoon "feels sick" when work keeps her away from her children.

The 'Water for Elephants' star - who has daughter Ava, 11, and son Deacon, seven, with ex-husband Ryan Phillippe - finds it hard to combine her career with parenthood, and insists she doesn't cope as well as people imagine.

She said: "There's this fallacy that we can have it all. I get it often, 'Oh, you have it all' - and I don't. It's hard.

"There are times when I take two years off and I don't work. And there are times I go to work and I feel sick because I can't see my kids for five days.

"And it's real hard to go, my work is important to me. The kids are important to me. My relationship is important to me. But all three of those things can't be functioning at 100 per cent at all times. You'll make yourself crazy."

Despite the challenges she is presented with, Reese - who recently married Hollywood agent Jim Toth - can't help but be driven towards seeking new challenges in her career.

The 35-year-old actress - who won the Best Actress Oscar in 2006 for her role in 'Walk the Line' - explained to UK edition of Elle magazine: "I'm a doer. If there's a task in front of me, I want to do it, get it done, get it done well, wrap it up and move on to the next task.

"I'm a worker, you know? When I started out, it wasn't a design that I wanted to accomplish a certain amount of things. But it was, like, finish that movie, it was a comedy, would really like to do a drama now. Do a drama, going to work really hard on this accent. What's my next challenge? I like to have challenges. That's fun for me."