That's fine if you want to do it that way, but he needs to gather energy somehow. It's the theory of equivalent exchange and all that. 

Obviously he must sleep in order to restore energy, but he'll also be needing another source of energy. The same goes for Cain. Whether it be by eating food, drinking water, breathing oxygen, or by some other semi-logical means (they're magical shadow beings, after all). It seems unreasonable to have them become practically immortal in that sense just because they are in a world where they can't assume their full physical form.

One thing I would suggest/call attention to.

In Twilight Princess, Rusl says this to Link: "Tell me...Do you ever feel a strange sadness as dusk falls? They say it's the only time when our world intersects with theirs... ...The only time we can feel the lingering regrets of spirits who have left our world. That is why loneliness always pervades the hour of twilight..."

I would say dusk/twilight could be a time when they are able to temporarily assume their full physical forms. Well... Cain could at least. I don't know the extent of the damage Magus inflicted upon himself. But, in theory, this would allow them to eat something or whatever.