People often describe screen control as a personality trait: disciplined people succeed, undisciplined people fail. That model is too simple. Most failures happen in predictable contexts: late at night, between tasks, after stress, or when the phone is already in hand.
A system performs better than a mood because it moves the decision upstream. The rule is chosen when the user is calm. The limit is visible before the session begins. The reward is connected to an action that has already been completed.
This is also why small contracts matter. A contract does not require a dramatic identity change. It asks for one specific behavior that can be completed, credited, and remembered.
The practical aim is to reduce the number of times a tired person has to negotiate with an app designed to continue the negotiation indefinitely.