Keyframing lighting intensities in Maya

 Below:
Live action footage and sky pre-comped (quick draft) and split out into an image sequence - targa file format - in Premiere.
The image sequence is then applied to the shot_cam image plane.
In doing this I can then see when the sky lighting levels change and can then animate the CG lighting levels to correspond accordingly.
The red highlighted rectangle shows the light 'key framed'.
Set the lighting intensity to the desired level, in the lighting attributes and right click, select 'key frame' to keyframe the lighting intensity.