Breaks are closely related to the two modes of attention, in which the brain can work. The first mode is a mode of concentration, the so-called central-executive mode, the second one is mind-wandering mode. The latter one is activated when reading literature, admiring art walks or sleeping in the daytime. In this mode, 15 minutes allow you to "reset" the brain and feel fresh and rested. At this time thoughts simply arise in the mind incoherently, you don't control. You must force yourself to periodically shift to a "walk" mode, disconnect from the Internet and e-mail.
In addition, you probably have tasks that require a lot of time to perform, and the challenges that can take you a few minutes. It's no good to jump from one type of task to another throughout the day. It's better to set aside some time to check emails (for example, twice a day) and read all received messages at once, rather than go into the mailings after each notification.