Here\'s some ideas that involves food (I like food idea) but just tone it down a lot, this might even apply to mana. I borrowed this from Chrono Trigger. Also another idea of using drinking and tie it to stamina.
Eating food increases the food bar (maybe a simple number).
As long as the food bar is not zero, the HP regenerates slowly by itself. If the food bar is empty, the HP regeneration stops but HP does not decrease. Food bar decreases slowly over time by itself. And if HP isn\'t at the max, it decreases faster.
Drinking fluid increases the fluid bar.
As long as fluid bar is not zero, the Stamina regenerates. If fluid bar is empty, stamina regenerates at half the rate.
As for stamina...
There should be two value (I think this is what Havena is talking about).
A Stamina value, decreases when performing action and increase when stationary or doing just light tasks (walking, etc).
An Energy value (negative fatigue), the energy value decreases overtime.
A Stamina\'s max value is the Energy value. The Stamina value decreases when performing extraneous task. And increases when performing light or no actions at all.
The Energy value decrease overtime and by one-tenth the amount of stamina decrease. The decrease overtime rate increases when food bar and fluid bar depletes.
Basically, let\'s say Stamina decreases by 20 points, the Energy value decrease by 2 pts.
Energy value can only be regenerated by sleeping (fast regen).