I'm not an expert, but from what I understand this effect is due to resistance. As I learned in high school physics, there are basically 4 things that effect an objects resistance:
# The material (i.e. Copper conducts better than aluminum)
# The length (i.e. a 10 foot cable is more resitive than a 2 foot cable)
# The cross section (a thick cable is less resitive than a thin one)
# The temperature (A hot circuit is more restive, hence cooling fans in computers)
So that would mean that if the LED in the move were to heat up, the system would become more resistive and there would be less voltage turned into light.