U2 star Bono pours so much emotion into his stage performances, he suffers what his family call "gig lag" for days after he completes a tour.

The Irish rocker struggles to sing his songs live unless he is in exactly the right frame of mind - and it means an exhausting rollercoaster of emotions.

He explains, "I can't sing in the registers that I sing in. So I really have to step inside the song or else I'm not going to be able to hit some of those notes.

"It's not a glorious dedication to the art. It's just a necessity. I probably think if it wasn't that way, I might try to fake it."

But performing at 100 per cent every night takes its toll on Bono when tours end and he's back home with his family.

He adds, "In a U2 show there's a lot of different feelings you're going through.

"My family refers to 'gig lag', where the coming down off that can be unpleasant. I am as high as a kite the day after, but the next day I can really hit a wall."