Better Than Before: What I Learned About Making and Breaking Habits--to Sleep More, Quit Sugar, Procrastinate Less, and Generally Build a Happier Life

by Gretchen Rubin

the mere act of adding one coin to the heap strengthens a habit, just as each subtraction weakens it. So each coin is actually two coins: the healthy habit itself, and the protection and reinforcement of that habit. The habit of the habit is more important than the habit itself.