This stage is different from the first three discribed, as some psychologists include it and some do not. It only occurs some of the time. It occurs most often when a person is hitting a "mental block" to a problem, and finds that they must put the problem aside temporarily and think of other things. Psychologists hypothesis that during this period of time one of four things happens:
• the person continues to attempt to solve the problem unconsciously
• the person takes the time to recover from the mental fatigue created by struggling to find a solution
• a part of the brain continues to gather clues from the environment, and one of those clues contributes to the eventual solution
• the "mental block" is caused by the person thinking the same way about the problem, and incubation leaves some portion of the brain able to concentrate on other paths for solving the problem
  
All four of these possibilities are essentially hindrances in problem solving, which have their own section.