By driving often near my local airport, I deduced that runway lights are kept always on as soon as the day light or weather conditions require it.
I also suppose that it comes from safety reasons established in the past ad that's a worldwide standard.
what I don't understand is why, given nowaday's instrumentation both on ground and airborne, it's not possible to switch them off when no aircraft is ever going to even fly at cruise altitude around it. Here at LIBP (Labruzzo Airport, Italy) we hardly have more than 8 transits during winter so probably 80% of the energy spent by lights it's totally wasted.