Wizard Spell Memorization
Wizards must prepare all of their spells in advance. The time to prepare spells is 15 minutes per level (0th level spell count as 1/2 level), and in this process they precast the majority of the spell. Later when they are casting a spell, they are essentially finishing the casting. The number of spells they can have memorized at any time represents the limits of their ability to hold the magical energies of the spell without them slipping away.
If a wizard has time out of combat and access to their spell book they can cast a spell from it taking the preparation time + casting time, without needing a spell slot open.
When a wizard is dropped to 0 hit points or below their ability to hold the spell energies is negated and they fade away, so that when they are brought back to consciousness they have no spells currently memorized.