Like with computers, multi-tasking is a mere illusion for humans. There are not so many things one can do simultaneously at any given time. So we (computers and humans, especially when in front of the machines) have to simulate it by constantly switching from one task to another and then back again. But these context-switches cost time and energy, giving an upper limit of how well we can put up with more than one thing to do. And in contrast to machines our context switching is dependent on the complexity of the task we want to jump into and of its familiarity with the task we are trying to get out of. It is then not very surprising that if we cram in our daily schedule lots of different things, little gets done. What is surprising though is that neither the project planning tools nor any of the calendar programs allow to schedule for these context-switches in an easy straight-forward way: there is no benefit in trying to schedule as large chunks as possible for any given task, so they create the illusion that 5 times one hour is as productive as one times 5 hours work! So back to your schedule and tidy up!