Researchers generally define the creative process in two phases: divergence and convergence. In the divergent phase, we generate a lot of potential ideas, while in the convergent phase we evaluate these ideas and focus on the most promising.
Over time we get better at divergence as we compile experiences that we can call on, and it is during medium durations of divergence that the largest number of ideas are active, rather than during short brainstorming sessions.
What does this mean in the context of idea creation?
It means that some ideas are best left to their own devices. Occupying our conscious thoughts with other activities means that we are letting an idea find its own connections in our subconscious.