Ratio between consumption (reading, listening, watching) and production (writing, creating). Imbalance kills creativity.
If you only write and don't read, you run out of fuel. If you only read and don't write, you never create. Set a 1:2 input to output ratio.
Creativity is both a muscle and a garden — it needs training and nutrition. Without input you repeat the same ideas.
Stephen King reads 4 hours a day and writes 4 hours. Exactly 50:50.