I think they may only be lit if parliment is still sitting at night time / gone dark. Could be myth its something i must have heard at some point.
No idea who had them fitted so i'm out on this one.
is it parliament still being in sitting and more than two MP's are awake?
So the lights were fitted at the bequest of Queen Victoria so she could check when her Government were sitting from her rooms in Buckingham Palace.
Apparently they were seen as slackers even in Victorian times... and little has changed.