in the zone

When programmers shut out all forms of communication because they are entirely focused on, and extremely busy with, writing software (or slinging code), they are said to be "in the zone." 

It also refers to "doing what you want to be doing" or it can indicate a transition, such as "I'm not in the zone for marriage and kids yet."

Historical perspective: This kind of "flow" was first identified and studied by the psychological scientist Mihaly Csikszentmihalyi, who defined it as a state in which people are so involved in an activity that nothing else seems to matter; the experience is so enjoyable that people will continue to do it event at great cost, for the sheer sake of doing it.