Justin Bieber finds his documentary series ''uncomfortable'' to watch.

The 'Yummy' hitmaker is currently starring in a YouTube Originals documentary series titled 'Justin Bieber: Seasons', and the project's director Michael Ratner has revealed the star found it ''tough'' to watch back some of the scenes, as they were difficult for him to film.

Speaking about one episode in which Justin, 25, details his past drug use and health struggles, Michael explained: ''Once we watched episode five together and he was just staring at it and he said, 'This is tough ... There's power in weakness.' I thought when he said that, he said, 'The very reason why it's sometimes uncomfortable for me to watch, that is the reason I want it out there.' ''

Michael and Justin watched all the episodes together before they were finalised, and the director says they had ''deep conversations about why things were a certain way'' during the viewing process.

The filmmaker noted that Justin became ''emotional'' during certain moments, and claims he ''certainly broke down'' on more than one occasion.

He added: ''It was never a place of subjectivity or trying to make a Justin Bieber commercial. He was never trying to use this as a vehicle to push album sales. It was really about how to tell [his] story.''

Michael also spoke about his hopes for the documentary, as he said he wants the series to tell the story of Justin as a ''human being'', rather than a superstar.

Speaking to 'Entertainment Tonight', he said: ''He is the first global superstar, megastar of the social media era, and that has ramifications and consequences. I think he's doing a pretty good job. I think what makes him a global superstar is that talent. Paired with crazy ambition and work ethic. He just does it over and over again. And I'm watching this and I was just like, 'Wow. He's grown up and he's amazing.' ''