Figure 9.18: Sleep after training improved insight Subjects were trained, in the morning or in the evening, to follow a seven-step procedure for solving a type of mathematical problem. Then, they were tested, either immediately after training or 8 hours after training. The crucial measure was the percentage who discovered an easier, two-step way to solve the problem (achieved “insight”). As the graph shows, the 8-hour interval increased insight only if subjects slept during that period. An 8-hour interval spent awake, whether it was during the day or during the night, did not increase insight.

(Based on data from Wagner et al., 2004.)