It's amazing what a break can do. There he was in the IPL, proudly wearing the orange cap over hair he'd razored back to the bone. He was all business.
The role of ego in batting is misunderstood: ego is a defence mechanism. Hayden's was breached by the end of his Test career, his decline was mental as much as physical. The basic function of batting, unimprovably distilled to 'see ball, hit ball' by Virender Sehwag, was still intact, as the IPL let him show.
There's your answer as to the real demands of Test match cricket. Haydos was a bit like a boxer whose chin went before his punch.
Michael Vaughan has had a break too, but he has not come back like Hayden. He's almost slipped from the radar now. He's playing T20 too, but for Yorkshire, and not very well. His desire must be draining away, and the thought that his decline was caused by mental fatigue is going with it. There must a physical element to it, too, something he can't quite recapture. Once that's gone, it's gone forever.