Started an overnight render and watched it go through the first couple of passes, just to make sure I didn't miss anything obvious during scene construction, and then walked away, satisfied that the lighting etc was what I was looking for. It was just a test render, so I wasn't too concerned about it. But, come morning, I find that the picture has rendered out so dark I can't even make out what it is supposed to be! How is this possible? None of the light settings have changed, but more importantly, why would it suddenly turn dark somewhere in the middle of a render? This has happened two nights in a row now on the scene I am working on. Fixing it involved cranking the intensity of the light (I'm using a single radial, no falloff, intensity of 3, plaus a volumetric spot), then slowly lowering the brightness back to where it was originally. This scene is just about ready for final rendering, so I'd like not to have to this happen again. Has this happened to anyone else, or any ideas to stop it from happeneing again?

Try leaving the lights in the office on :) Seriously, though what changes in settings did you make between trial render and final render? there must be some clues in there somewhere

Nope, I didn't change a thing, at least, nothing could account for a change in the lighting. I added a few objects, and tweaked the location of a few more, but nothing else. None of the objects I added are interfering with either of the lights. I stumped. When I started the render last night, everything was well lit. So something happened during the render, as far as I can tell, that screwed things up.

I meant literally what changes in the render settings, i.e change of render size, switch to premium render, true ambience, reflection, ray depth etc etc?

No, I didn't change any of the render or document settings.