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Fullness is one of the four main player statistics. It is important to keep your Fullness high as it is directly proportial to how high your health can naturally regenerate. Over time your health will increase up to an equal level to that of your Fullness.
Fullness will increase over time, which is seen as your Fullness meter gets lower. Moving around, swinging tools, and punching will all lower your Fullness faster. If your Fullness reaches zero, you will begin to take damage until you die, or until you eat food.