Johnman
I think you will find that the topic of cockpit lighting at night is a quite contentious and thorny old chestnut! Pilots tend to have very strong views, one way or the other. On short trips turning the lights off or down shouldn't have much of an effect but on medium and long haul it is a different story.
The USAF did quite an extensive study into cockpit lighting and the copy I saw was in a doctors office in London. The report states that, by leaving the cockpit lights as bright as is reasonable during the cruise phase, the onset of fatigue can be reduced by up to 75% as the natural melatonin, (which helps you to sleep), in the body is less likely to be released than if the surroundings were dark. It should also be remembered that unless actually focused onto an object then, at night, the individuals eyes are naturally focused about nine inches the other side of the glass of the windscreen.