A. By 'sleeps with the light on,' I am assuming you mean that she is sleeping with her regular, bright, bedroom light on and not a dim night light, which shouldn't cause any problems...
The only real harm would be if it were interfering with her sleep. If she sleeps well and is well rested the next day, then it may not be a big issue.
Experts are recognizing that not getting enough sleep can have significant effects on children. In addition to appearing sleepy, they may have a short attention span, be hyperactive, or irritable.
Is she having any of these problems?
You are right that most people should sleep in the dark or in a dimly lit room. The reason is that melatonin, a natural hormone that our body produces and which helps stimulate our going to sleep, can be inhibited by bright light.
Here are some websites where you can get more information about melatonin and sleep: