Simply put, aging is a bitch.
I can't pull all-nighters or concentrate as hard as I could in my youth anymore, among many other things, and I'm more or less constantly exhausted simply due to the fierce but uncaring passage of time.
>mental rather than physical
No doubt it's more mental than physical, but it all comes down to aging one way or another. There's less wonder in life the more I experience; I'm just tired and the only explanation is I'm old, full stop. Most people are more young at 35 than I am, but I'm not one of them.