“Interruptions produce a heavy mental workload. For example, if we were reading, an interruption means we have to find our place and rebuild our mental structures to resume. If we are involved in deep concentration and mental activity – such as might be required in programming, writing or design – the disturbance that results from an interruption can be even more extreme. The psychological literature is filled with studies demonstrating the high cognitive workload caused by interruptions and the resulting inefficiency with which tasks get completed. The research literature on the performance of tasks shows that interruptions lead to errors: people forget where they were, sometimes resuming by repeating a task already done or by skipping a step not yet done. Both can have serious negative consequences. In addition, when tasks interrupt one another, each gets done more slowly due to startup time. The total time taken can be far greater than if none of the tasks were interrupted.”
Living with Complexity, Donald A.Norman