Stress leads to cognitive tunneling because it causes your brain to concentrate its resources. By reducing stress, you decrease the likelihood that you'll end up with tunnel vision. Workload is one of the most common workplace stressors, causing you to ruminate on questions you aren't sure how to answer. To reduce stress, proactively answer these questions.
The most productive people do what they feel is most important more of the time than anyone else. They avoid being dragged along by subconscious forces that constantly redirect their attention. Check email when you intend to — when it is the highest priority — and stay out of your inbox the rest of the time.