History suggests that a burst of creative inspiration, or even the solution to a baffling problem1, can spring from the unconscious work of slumber2.tMendeleev3 credited his discovery of the periodic table4 to a dream that showed him where to place the elements. Friedrich August Kekule discerned the ring shape of benzene in a somnolent vision of a snake biting its tail5. While these might seem like exceptional cases, research confirms that a good night’s sleep can open the door to insight.tIn a study published in Nature this year, German researchers trained several groups of students to perform a memory task. Each student learned two rules for converting a string of eight numbers into a new string. A third, hidden rule would have reduced the steps in the calculation, allowing the students to solve the problem immediately. The groups were tested once after training and then again eight hours later. History suggests that a burst of creative inspiration, or even the solution to a baffling problem1, can spring from the unconscious work of slumber2.tMendeleev3 credited his discovery of the periodic table4 to a dream that showed him where to place the elements. Friedrich August A review of the ring shape of benzene in a somnolent vision of a snake biting its tail5. A third, hidden rule would have reduced the steps in the calculation, allowing the students to solve the Problem immediately. The groups were tested once after training and then repeated eight hours later.