It's because most Casio models that have a "power saving mode" or "sleep state" also have "auto EL," which is triggered by a tiny gravity powered switch inside the watch -- turn the watch a certain way towards your face to read the time, and the switch makes the light comes on (if the solar panel also decides the room is dark enough to need the light) and/or makes the watch "wake" from "power saving mode."
Probably a good guess, but extremely dim light isn't going to be enough to charge it appreciably. Normal indoor florescent lighting is going to take about 90 times longer than sunlight to accomplish anything, and the dimmest of lights is probably barely going to "break even" against Casio's estimates of "normal battery use." Here's the chart for the GW-S5600:
I just charge my solar watches outdoors, in the sun, when they drop to "medium" or below. It's important to note that in any of Casio's charts, the largest number in a given line is "how long it will take to charge the watch from the bottom of 'medium' to the bottom of 'high'" and the last number in a given line is how long it takes to charge the watch from "the bottom of 'high' to 'fully charged.'"
