Keira Knightley has explained why she was so exhausted while filming political thriller Official Secrets - she was also sleep training her daughter.

The actress, who portrays real-life Iraq War whistleblower Katharine Gun in the film, was up all night playing mum with Edie when she wasn't on set - and she admits she found it hard to balance motherhood and the intense role.

"Any actresses out there, do not move the child from the cot to the bed when you’re just about to play a lead role in a film that has a lot of words in it because remembering them is quite tricky," she tells Page Six.

But the star, who is currently expecting her second child with husband James Righton, decided to use the intense exhaustion for her performance.

"I felt very on edge, but for different reasons. So I used it all," she adds.

Knightley previously discussed her struggle to be an attentive mum while her little girl was up all hours, telling WENN her daughter was up five or six times a night between midnight and 6am.