Having worked on a team that was responsible for uptime of some high traffic web apps, there's a difference between when you're working and when you're on call.

Having worked on a team that was responsible for uptime of some high traffic web apps, there's a difference between when you're working and when you're on call. We had an on call schedule for emergencies and down time, but we rarely had actual emergencies to respond to. The rest of our work was done in two week sprints. No one cared when we did the work, as long as we stayed on track with those two week sprints and got everything done most of the time.