In the end, Shiro ended up explaining how food needed to be digested and all before casting her healing spell.
She could cast the spell despite how tired she was.
But doing that would strain her body and potentially shrink her muscles while also risking organ failure to happen.
And since her body was fine and she only lacked vitality, it was much harder to recover with magic with her body in such a state.
Danzel was somewhat knowledgeable of this fact too, so he simply nodded at her.
He remembered reading in one of Velkir's books of necromancy went about how repairing an undead body wasn't as efficient as the living.
Which made healing skills for the undead quite taxing in mana.
But on the plus side, compared to that minor weakness, they didn't have to restore flesh like the living, which cost mana too. At least that was the case for the skeleton-type undead.