The USAF have done a mountain of research on this subject, (as well as pre-flight rest and meals, in-flight rest and meals etc.) and their conclusion was that fatigue could be reduced by as much as 75% by having a bright flight deck in the cruise sector.
Bear in mind also, from the medical point of view, at night, unless you are actually focused on something, like CBs, another aircraft etc. then your eyes are naturally focused only nine inches on the other side of the windscreen, not much use at all really!
Rule of thumb, unless you are looking for, or avoiding weather, or over Africa and similar places then a bright flight deck during the cruise is a good idea.