Mila Kunis fell asleep in a sex scene on 'Friends with Benefits'.

The Ukrainian-born actress stars alongside Justin Timberlake in the Will Gluck-directed romantic comedy, and she admits the pair had to do so many naked scenes during filming she eventually became tired and needed to nap.

She said: "Towards the very end, we were shooting the lovemaking scene at his father's house. It was like an 18 hour long day and 17 hours into it, I was laying there and he was kissing my back and I was like, 'I'm just going to close my eyes for a minute.'

"Next thing I know you're like, 'OK, we're done!' I literally took a nap!"

Justin - who plays the male half of a couple who decide to eschew a normal relationship in favour of a Friendship with sex involved - reveals they had a choice to do the sex scenes at the beginning or the end of the shoot, and the pair chose to do it at the end because of the amount they had to do.

He told Total Film: "We wanted to push it to the end because you're like, 'Oh man, we're going to have to do a lot of stuff.'

"It was two five-day weeks with a couple of days to just try stuff out. Some of it made the movie."