Brightness problem

Okay, I think this started happening after updating to 10.6.8. But whenever I take a break from my laptop, I put the brightness to zero and leave the laptop to 'sleep' without putting the cover down. However, sometimes the brightness just turns up automatically by itself and then I have to press the 'lower brightness' to make it zero again. Why does this happen? And does anyone know how to fix? Wasting my battery.