The Black Swan actress claimed to have been so exhausted by the rom-com's hectic shoot that she needed the opportunity to refresh during a passionate moment with her co-star.
"Towards the very end, we were shooting the lovemaking scene at his father's house," Kunis explained to Total Film. "It was like an 18-hour long day and 17 hours into it, I was laying there and he was kissing my back.
"I was like, 'I'm just going to close my eyes for a minute'. Next thing I know, you're like, 'Okay, we're done!' I literally took a nap!"
Timberlake previously accepted that although his sex scenes with Kunis felt "awkward", they were essential to making their Friends with Benefits characters "affable and charming".
Friends with Benefits is currently on US release and is due to hit UK cinemas on September 9.
> Justin Timberlake on 'Friends with Benefits': 'My butt is comedy gold'
Watch the trailer for Friends with Benefits below.