Hypergraphia: The driving compulsion to write; the overwhelming urge to write. Hypergraphia may compel someone to keep a voluminous journal, to jot off frequent letters to the editor, to write on toilet paper if nothing else is available, and perhaps even to compile a dictionary. Hypergraphia is the opposite of writer’s block.
So, there’s a name for it. I never knew that. 🙂
From: here.