Officially at Easyjet, lights can be set to dim 2. In effect that is actually quite dim and one set of lights (either window or ceiling lights) could be switched off making the cabin fairly comfortable for sleeping but still light.
Me personally? I make it like the black hole of Calcutta but regular cabin patrols, as they always should be, need to be made to check on everyone. One thing to bear in mind though, it has to pretty late. Typically, I won't turn lights off on anything that lands before 11pm....even Midnight.
It is generally the crews own preference and choice. Few of my colleagues tend to have lights on bright on longer distance late flights.