Natural light comes from fire and sun, both supplying heat.  Our biology needs thermal energy as well as light; innumerable biological functions depend on both.  Today's artificial lighting (LED, fluorescent), on the way to eclipsing the incandescent bulbs we grew up with, is not full spectrum and creates what is known as biological darkness.  Dr. Alexander Wunsch is a German photobiologist who is interviewed below by Dr. Joseph Mercola.  Listen for a fascinating education on why the sun is so important to living things:

Info on Dr. Wunsch and his practice here