He's concluded that his health issues mean he's not able to do what's required of him - and admits he's "indescribably sad" to say he doesn't know if he'll ever be back.

Wintersun drummer Kai Hahto has been drafted in to record the follow-up to 2011's Imaginaerum, but no decision has been taken over who'll be behind the kit when the band hit the road.

Nevalainen says: "For a number of years I have suffered from occasional but very difficult insomnia, my unwanted companion during many of our tours. I've done my best to try to control it, or at least to cope with it. There's not much information available on why some of us suffer from sleeplessness. There is no 100% cure for the problem.

"During the last month it has become evident that due I can't fulfil my duties as a musician in the way that I'd want, and with the precision the music deserves. Thus I have decided to step aside from Nightwish during the recording of the forthcoming album and the subsequent tour."