Narcolepsy

Whenever you use a Class Feature/Ability, or cast a spell of your highest level, roll a Constitution saving throw. (The DC is 15 minus your current character level)

On a failed save, roll 3d8. If this total is greater than your current hit points, your character falls unconscious. Your character falls unconscious for 1d4 minutes, they take damage, or someone uses an action to shake or slap them awake.

The number of d8 you roll for increase by one for each level of exhaustion you have.