Edison used to sit with marbles in his hand on a chair placed on a metal tray. During deep thinking, he used to enter into a hypnogogic state. Just at the point of entering into sleep, he would get solutions for his problems. The marbles would slip out of his hand, drop into the metal tray and make a noise to wake him up. At that moment, if he did not wake up, the solutions he got would fade away as dreams.