Why does a light bulb activate an infrared sensor? Is there a way I can filter this out? I've made 3 programs. One is written in python, c++ processing and bash. 

Everything seems to be okay but then sometimes I get a signal from the arduino that activates the function to lower down the volume on the python code listening for serial data. This is sometimes random but it seems that I can reproduce the same behavior when I turn the light on. I can cause the same disruption if I wave my hands over it quickly.

How can I filter this? I know that IR is part of the electromagnetic spectrum in which the visible light takes part. The sensor is facing upward for a type of omnidirectional receptor. I've placed a TV IR blackish cover over it but with the same results. Do I need to use the function to hash decode the incoming signal? I suppose that it needs a simple circuit as any other sensor.

I want to discard the possibility of a random activation in my code.

Edit:

Taken into consideration the answers until now I can confirm that this is part of a physical phenomena due to the distance between my light and the device. 

As for the solution to my volume problem It seems that I used "0" in the same statement as the ir code I remapped in the python code. It turned out to be part of the IRlib library of the arduino code that returns a "0" when it encounters an unknown signal. (due to protocol specification? - out of useful range? )

The thing is that I can change the if statement value for other than "0" but that won't stop it from returning "0" and that's a disadvantage. The solution that came to me is simply intercepting the signal and use continue to skip this. 

Is there any alternative to this?
Yes, a light bulb will put out some infrared light. 

Decoding remote control signals isn't that hard - there is a tutorial at instructables - http://www.instructables.com/id/The-Easiest-Way-to-Use-Any-IR-Remote-with-Ardiuno/?ALLSTEPS

This also means you can tell the difference between different buttons.

P.S. - when posting in this forum, it helps to include your code and your wiring.