Steven Spielberg was left "sleep deprived" shooting 'Ready Player One'.
The 71-year-old filmmaker admitted his latest sci-fi saga has been one of the most "difficult" projects he's ever worked on and he needed to learn a lot of new skills to make it work.
He said: "It's the third most difficult film I've made after 'Jaws' and 'Saving Private Ryan'.
"I think there is a distinction between a physical challenge, which was both 'Jaws' and 'Saving Private Ryan' and another kind of challenge, which is trying to do something technologically new that has never been done before.
"Going from a live-action, real-world production - with sets and props and extras and chases and all kinds of rigmarole - to digital capture in a volume (performance capture stage), sometimes with just a day in between, created a level of effort I ha never experienced before.
"I had to learn how to make a film like this while I was making a film like this. And that meant a lot of trial and error.
"I was only getting four or five hours sleep a night, because I Was always thinking about what I was going to do the next day. So it was maybe the most sleep-deprived experience I've ever had making a movie."
The acclaimed director admitted he wondered if it would have been an "easier" movie to make when he was younger, but he thinks it needed the level of experience he has now to pull it off.
He said: "At the same time, it was one of the most fulfilling experiences I've had.
2I made the movie when I was 69 years old. I had to ask myself, several times, would this have been any easier if I had been 29 years old? I think it was so complicated I wouldn't have had any clue what to do at 29.
"I had a lot of experience that gave me confidence to say, 'This is the time to tell this story'.
"I probably would have been just as tired at 29 as I was at 69. Puppies get more tired than old dogs. They run around and fall asleep.
"Old dogs measure their distance a little bit better - based on the experience of being an old dog."