Traffic lights are not activated by the weight of a vehicle waiting at the intersection. They operate pretty much on the same principle as a metal detector, using RF to detect vehicles.
I find that if, as you approach the intersection, you weave around a bit over the detector (you can see where this is by the cuts in the road - generally a square figure of eight type shape), so that you maximise your chances of being detected, nine times out of ten it works. If you ride the same route regularly, you will learn which sets of lights are the one out of ten that no matter what you do, don't detect you.
As for stopping at lights early in the moring, most of my rides begin prior to 5am, weekdays and weekends. My rule is fairly simple - if there are cars around, stop at the lights and either wait for the light to change or wait until the car is gone. If I there is only one or two cars around, I delibrately miss the detector and go through the red light once the cars are gone. If there are no cars around, I can't see the problem with going through the red light, although this is still illegal. My justification for this is that at this time of the morning, normally there are only one or two cars around at a time and I reckon that some motorists would be more annoyed at being the only car to have to stop at traffic lights just to let a bike through than for a bike to go through a red light. Does that make sense?
As for crossings where there are bicycle signals (little green bike next to the little green man), my assumption would be that these occur where a shared pathway crosses a road. In that case, I think it would be legal for a cyclist to ride across the road, since the crossing is simply part of the shared path.