In a series of experiments published in 2011, they found that having an uncompleted goal generated intrusive thoughts during completely different tasks and impaired performance on problem-solving exercises. The simple fact of having something pending (unresolved, undecided, or not written down anywhere reliable) consumed mental resources that should have been available for something else.
Here is, perhaps, the most useful finding in this entire area of research: in those same experiments, all the participants had to do was formulate a concrete, specific plan for tackling the pending task (not to complete it, but simply to decide when and how they would do it) for the intrusive thoughts to disappear and their performance to return to normal. The mind did not need the task to be completed. It needed it to be decided.
— Read on facilethings.com/blog/en/decision-fatigue