It's because of the intensity of the light. Conditions of both unusual brightness or darkness increase the body's use of vitamin A. Polar bears store so much vitamin A in their livers that it's dangerously poisonous to eat. I dont remember the toxic levels but they're well above general requirements, because A's fat soluble the liver can store quite large amounts so for a normal life style I reckon a pound of pig's liver a month is plenty. It's a long time ago that I looked into these things so you might want to check if you're planning more than half a pound a day.