Our sun is a star - and one upon which we are physically dependant upon for life itself. Its heat and light are what make life possible. Should the sun burn itself out, we would not exist, because the cold and darkness of outer space surrounding our planet cannot sustain life without it. Light and life go hand in hand. We already know first hand what it's like when the sun doesn't come out for an extended period of time.
People get gloomy and even depressed.
When the days get shorter, people complain about having to commute in darkness. There is a village in the northern-most part of Norway where the sun does not rise above the horizon from November 25th through January 21st. Even those who are emotionally healthy the rest of the year may become tense, fearful, and preoccupied. In this village, on January 21st the celebration begins. The season of Epiphany is somewhat like that town's celebration.