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Interruption studies typically focus on external interruptions, even though self-interruptions occur at least as often in real work environments. In this article, we therefore contrast external interruptions with self-interruptions. Three multitasking experiments were conducted, in which we examined changes in pupil size when participants switched from a primary to a secondary task. Results showed an increase in pupil dilation several seconds before a self-interruption, which we could attribute to the decision to switch. This indicates that the decision takes a relatively large amount of time. This was supported by the fact that in Experiment 2, participants were significantly slower on the self-interruption blocks than on the external interruption blocks. These findings suggest that the decision to switch is costly, but may also be open for modification through appropriate training. In addition, we propose that if one must switch tasks, it can be more efficient to implement a forced switch after the completion of a subtask instead of leaving the decision to the user.