If you use the Power Saver Mode to switch off wireless at night, then you may not realise that it no longer works since the recent automatic firmware undate. It will switch off when you want it to, but will spring back into life just after midnight.
I contacted BT and they said they were aware of the problem which is affecting all customers. A new firmware will be released by the end of July to rectify the issue. They, and their forum suggest resetting the router to default settings, but this rarely works.
The blue lights use very little energy anyway, so might as well leave it on until the next firmware release. The guy told me that I could unplug the router at night, but I don't really see the point of that (though tree-huggers might say otherwise).