I just got back from a one hour hike in the hills near my house. While I was hiking, I had my phone with me. According to telephone records for the last 10 weekdays, I received 2 text messages and 1 phone call (on separate days) between the hours of 2 pm and 3 pm. So during my hike, the probability of interruption was roughly .3. If I used a service to receive notifications of new email on my iPhone, the probability of interruption from incoming email would have been about .93 (based on my email records from 2008). In my typical office environment there are also possible interruptions from IM, Twitter and kids. The probability of receiving an interruption in my office during that hour approaches 1.
Subjectively, the probability of interruption strongly influences my stress level. A low probability of interruption encourages relaxation while a high probability causes stress and reduces concentration.
Why does the probability of interruption and not the interruption itself seem to cause stress? I think it's because I anticipate being interrupted. For example, when I travel for work, I often leave my house early in the morning to catch a flight. As I go to bed the night before departing, I set an alarm (a planned interruption) to wake me up. I often wake up throughout the night anticipating the upcoming alarm even though it hasn't gone off yet. I lose sleep without the interruption actually happening.
Another anecdote: Several years ago, I worked in a traditional office environment. While at my desk, I could be interrupted by an email, a phone call or a coworker across the room. When I left my desk and walked down the long hallway to the rest of the building, I always felt a release of tension. As I rounded the corner to pass another coworker's office, I felt an increase in tension. It was as though I was walking the hills and troughs an interruption probability function.
Probability of interruption may explain why some people are most creative when in the shower or on the toilet. There's some indication that stress reduces creativity. Because of social taboos, the probability of direct interruption while bathing or defecating is practically 0. That reduces stress and allows one to be creative.
I suspect that intentionally limiting possible interruptions can increase one's creative throughput.