The iMac G5 (Ambient Light Sensor) contains an Ambient Light Sensor on the underside of the display bezel that adjusts the brightness of the sleep indicator light according to the level of the surrounding light.
In a dark environment, the sleep indicator brightness decreases; in brighter conditions, the sleep indicator brightness increases. With this behavior, you can tell if the computer is asleep when the room is bright, but the computer's sleep indicator light won't keep you awake at night.