Green Day frontman Billie Joe Armstrong has hinted that it was the stress leading up to his 40th birthday in February (12) that may have sparked the meltdown which forced him into rehab last month (Sep12).

The 21 Guns singer checked himself into a treatment facility in a bid to tackle ongoing substance abuse issues after losing his cool during a gig in Las Vegas, when promoters attempted to force his band offstage by signalling the rockers had one minute left.

Armstrong stopped the show, hurled abuse at the gig organisers, smashed his guitar onstage and then stormed off.

He has been in rehab ever since, forcing the trio to scrap a series of promotional appearances and an upcoming set at the Voodoo Music + Arts Experience festival in New Orleans, Louisiana.

And it appears his problems may have started at the beginning of the year (12) - as the Green Day frontman fretted about turning 40 while mourning the loss of a good friend, called Brando.

In one interview for Nylon magazine, recorded before the Las Vegas gig and the rehab spell, Armstrong states, "For three months before it (turning 40), I was on the verge of a nervous breakdown. Then, when it happened, I was like, 'Oh, I'm still alive.'"

His bandmate Mike Dirnt has offered up a more serious account of why Armstrong has sought help, explaining the rocker is burned out after recording three albums back to back.

He admits the trio was in need of a break as the prospect of promoting new releases Uno!, Dos! and Tre! loomed: "It was a tremendous undertaking... (and) it catches up with you a little bit. We definitely jumped off a moving train. All that inertia. There were signs of things hitting the fan; we hadn't slept in forever."