I was working with the US navy in the late 1970's when the USAF conducted trails with regard to use of cockpit lighting at night and fatigue. I can back up Parabellum's statement that their conclusion was that the use of bright lights reduced the level of fatigue markedly and was strongly recommended.
As for the having the dome lights on DIM at least on Airbus aircraft, this is to ensure minimum area lighting in the event of an Emer Elec Config situation where only the right dome light will be available.