Why are cabin lights dimmed?

USATODAY.com: "Depending where in the world you were flying, the other aircraft could have been as close as 1,000 feet vertically from you. Domestically, at least for now, aircraft must be separated by 2,000 feet.

For instance, our choice of altitudes in one direction might be 31,000 feet, 35,000 feet or 39,000 feet (33,000 feet, 37,000, 41,000 feet, etc., are reserved for opposite direction traffic).
A new program, Reduced Vertical Separation Minimum (RVSM), should start domestically in the near future. It allows planes to match international guidelines because of advancements in technology both in the aircraft and on the ground.

It will give pilots more flexibility in finding an altitude that gets around wind and weather. We'll have our choice of 31,000 feet, 33,000 feet, 35,000 feet, etc. Opposite direction traffic would fly at 32,000 feet, 34,000 feet, 36,000 feet, etc."