How to remove dullness of mind, without sitting meditation, in regular activity?

When you have to perform a certain task, you can't meditate during that time, but mind needs clarity. I feel very dull during that time, I want to increase my awareness and alertness in that performance time. So, how I can remove dullness of mind (while performing a certain task during which I can't meditate)? What are the causes that lead to dullness?