Do spirits in flesh form have to eat, sleep, use the bathroom etc etc? Or is it only optional for them, or do they not have to on anything?

There is nothing officially stated on this, so it is entirely the GM's call.

I have always unofficially ruled that the Regeneration power came with a lack of need for normal food. That is, maybe you still suffered from a lack of food, but it was damaging you so slowly compared to the speed at which Regeneration heals you that you'd never know. That means, for example, that Vampires don't die from lack of hot dogs and Master Shedim don't immediately croak from the fact that the bodies they bring back to life don't have nearly enough nutrients in them to keep them going.

But I would note that a spirit inhabited into a scarecrow will burn, I see no reason why a spirit inhabited into a live human would not starve. It's a physical limitation of the host that is in no manner directly compensated for by the spirit's magical powers.

That's not an official stance or anything, I believe that the official stance really is "leave it up to individual game masters" - but it is my stance.

-Frank

*nods head* That makes sense. The way my group chose to do it thus far is a middle ground approach. Eating, sleeping, and all that are considered habits. So the spirit tends to do these things when it can, but it is not required too if it does not want too, or has been given a taste where it needs to say.. stay awake for days etc etc.

Good point about the regeneration, didn't even consider that.