The Irish singer cited exhaustion as one of the key factors for his departure from the boyband in 2004 and warns the Baby hitmaker to learn from his story and slow down before it is too late.
He tells Entertainmentwise.com: "I think with One Direction and Justin Bieber, the difference is, there's five of them boys (in One Direction) so, one of them might have a bad day but the rest of the four can keep it going.
"But Justin just seems to be constantly going, working so hard, so someone's going to have to tell him to take a break soon. That's what happened to me. If I'd been able to take a break before Westlife, I probably would have still been in Westlife.
"You have to be in control of it and take the time off and enjoy it because you don't want to get to the stage where you're not enjoying it anymore. This is the best job in the world, but as long as you appreciate what you're doing."