In the paper by the “turn your clocks” reminder was a note that during the world wars daylight saving was followed in the winter.
Did it do any good, or was it just another of the wartime programs to make everyone feel involved?
If it was effective, why was it discontinued?
I can’t answer the question, but I can sort of hijack it with a related followup. 
I’ve seen some WWII references to things in England as taking place in “English Double Summer Time”, the implication being that they set their clocks two hours ahead during the summer.
True? Same idea as in Poirot’s question?
I don’t know if it did any good during the world wars, but it was also tried again during the energy crisis of 1973. I happen to know quite a bit (more than you’d expect, anyway, seeing as how it was two years before I was born) about this particular year because my father had a whole bunch of old newspapers of this vintage out in the garage and I used to sit out there and read them. Some good comics back then, I must say.
Anyway, when I found out about the year-round daylight savings time, I asked Dad about it. He said it was terrible. In the summer, there is extra daylight to go around so it’s all right to give some extra to the evening hours. However, in the winter, the morning (at least in Montana) needs all the help it can get. He said that it was rather dangerous for kids to wait for school buses or walk to school in the dark. He’s a bricklayer, so working outdoors in the dark would be a big problem, too. Therefore, the hours of his job were shifted an hour ahead, beating the energy-saving purpose for our family, at least. I got the impression that some schools shifted their hours ahead as well, for safety reasons.
Perhaps it worked better in more southern latitudes, and it may have saved some energy, but it seems there were enough drawbacks to the plan not to continue it year-round once the crisis was over.
I think that there are just too many variables to account for the effect of daylight savings time. I suppose you could measure peak levels and see if the time change evened them out. That would be the main effect.
I don’t think that lighting is using up as much electricity in southern latitudes as A/C does.