Luminous paints such as those used on watch dials make use of chemicals that can absorb visible light and re-emit it over a protracted period. This phenomenon was first noted in the 1500s when stones were discovered near Bologna in Italy which glowed after having been exposed to sunlight. They would glow in the dark for six years after heating intensely in the presence of carbon black. As one can imagine, this attracted alchemists and anyone who wanted to dabble in mysterious arts. The stones turned out to be barite, or barium sulfate. Heating with carbon converted the barium sulfate to more luminous barium sulfide, which came to be known as 鈥渓apis solaris鈥�, or sunstone. Since then, many other compounds that store light have been found, with calcium sulfide probably finding the most use in theatrical effects and luminous dials.
Whether the watch dial glows or not depends on the intensity of the light to which it has been exposed during the day and the amount of time it has spent in the darkness. The glow fades as time passes. This phenomenon is of course amenable to many applications. Such as the one devised by a clever fourth-grader who won a prize at an inventors competition with a glow-in-the-dark spray-painted toilet seat so that people wouldn't have to stumble around in the dark in the middle of the night. Sound like a great idea. Men can find the target easily, and women can tell if the men have left the toilet seat down!
