Unlike Strength, Agility, Health, and Magic: Mana and Stamina stats were consumable type stats. This did not mean that the Mana and Stamina of an existence dropped, it was simply consumed from its maximum.
To recover Stamina and Mana, one needed to rest properly, and depending on the type of rest and the circumstances, the recovery speed would be affected positively or negatively.
When an existence exhausted too much Stamina, they could walk around and slowly recover their Stamina. However, if such an existence stayed completely static, that recovery speed would be far higher and if they slept, thousands of points could be recovered with one night's sleep.
Mana was similar.
To activate magic spells or skills that required Mana to show their effects, the user needed to have the necessary amount of energy just as they needed Stamina to run at high speeds or fight hard battles to the death.