Justin Timberlake spends "14 hours" at a time in the recording studio.
The singer-and-actor penned some tracks with British star Jessie J and the 'Do It Like a Dude' hitmaker was astonished by his work ethic.
She said: "When he's in the studio, he is in the studio. We would literally spend 14 hours in there and not leave.
"I would be like, 'Where is the toilet please?' I'm going to wet myself."
Justin discovered Jessie after witnessing some of her teenage performances on YouTube and she was overwhelmed by the praise he heaped on her.
She recalled to Glamour magazine: "Justin sat me down and said, 'You have the most incredible voice I've ever heard.'
"When someone likes that says it to you, you want to do one of those dance moves and sing, 'Love and sex and magic.' "
She said: "I went round the UK and tried to get signed, and at the time it was like the bombardment of women - VV Brown, Gabriella Cilmi, Cheryl Cole, Adele, Duffy. Everyone was going, 'We're not going to buy into another girl now.'
"I got signed as a writer by Sony, so I flew to the US and went, 'Maybe I'm not an artist.' I knew at the back of my mind that I was, but sometimes you need someone to go, 'Come on!' "