[AmericanChopper]

Most banks have a window of a couple of hours per night where access to the core system is restricted for settlement. It's kind of a necessity of the business model. However most banks (including every bank I've ever worked at, which is quite a few), don't shut down services during that time. ATM, credit and debit cards, everything else... All those systems run all the time. They just have an eventual consistency model. The compromise that the banks tend to make, is that certain types of fraud are more tolerated during that window, rather than sacrificing availability.