The modern knowledge worker's workday schedule is fraught with peril. This peril usually manifests itself in the form of meetings.
To combat productivity losses from meetings, we like Paul Graham's advice in following the maker's schedule in the morning until about 2 or 3 PM. And then the manager's schedule in the afternoon from about 2 PM or 3PM until 5 PM by setting "office hours." This is an application of the 80/20 rule. We allocate approximately 80% of our workday to maker functions and 20% of our workday to manager functions.
Of course, the hard part is having the discipline to stick to this schedule. But a company culture help to entrench this practice.