In order to reach maximum productivity, you need to focus on the task at hand. However, you cannot stay fully focused for an entire 8 hour (or longer) working day. I suggest you to split your working day into three different kinds of time, detailed as follows:
Work mode: In this, you work on small tasks that do not require deep concentration. This might be fixing a misaligned user interface, simple refactoring, moving accidentally hardwired strings into a resource bundle, and similar tasks. Handling e-mails can also be done in Work mode.
Focus mode: In this, you work on complicated stuff that requires your full concentration. This might be programming a Java method or debugging a difficult issue.
Recharge mode: In this, you do something else to revitalize your...