If it is all background lit lights like gauges, led displays in the bottom etc. it might be the automatic gauge brightness control. I noticed my E46 seemed to change these sometimes rapidly and without a reason but haven't yet in my X3. I think it even adjusted the business radio display at the same time.

In the light switch unit there is that small clear / greyish dot (photo cell?) that is measuring the amount of light. If you cover that during daytime with your finger you can see the brightness starts to change slowly. You might need to have your lights turned on or in automatic mode, not sure. If they are manually turned off I think this feature is disabled and brightness can be only adjusted manually using the roll knob. This could be playing tricks.

For example on a bright sunny day if you drive under a birdge or something like that you can see it adjust them and when you come out there it may take a while to adjust them again. Usually it is stepless but sometimes jumps from bright to dim and back. Now it could also happen if you lean on your left elbow towards the door and your hand creates a shade on top of the measurement unit.

If you loose some other lights like high beam or warning lights that is a different story.

- p