Psychologists have identified a state of mind called flow in which we’re capable of incredible concentration and productivity.
The importance of flow to programming has been recognized for nearly two decades since it was discussed in the classic book about human factors in programming Peopleware: Productive Projects and Teams by Tom DeMarco and Timothy Lister (Dorset House, 1987). The two key facts about flow are that it takes around 15 minutes to get into a state of flow and that even brief interruptions can break you right out of it, requiring another 15-minute immersion to reenter.
DeMarco and Lister, like most subsequent authors, concerned themselves mostly with flow-destroying interruptions such as ringing telephones and inopportune visits from the boss.
Less frequently considered but probably just as important to programmers are the interruptions caused by our tools. Languages that require, for instance, a lengthy compilation before you can try your latest code can be just as inimical to flow as a noisy phone or a nosy boss. So, one way to look at Lisp is as a language designed to keep you in a state of flow.
(From the online book, Practical Common Lisp)