Each one of us is composed of a body and a soul. The body is dark, the soul is bright. When we subdue the body and put it to work for the soul, the body is consumed and 'becomes' light, just like the wick of a candle.
The most natural thing in the world is to sleep at night when it's dark, and awake with the sun and the return of the light. During this time of year, as the days grow shorter and the night encroaches, we prepare for the New Year, and the Day of Judgement.
We attend slichot either late at night or early in the morning, in the thick of the darkness. We treat the night as if it is day. When we go to say slichot we are saying to HaShem: "Look, we treat the night, Your creation, as if it's day, treat our 'dark' actions as if they were 'light.'"
We're asking HaShem to redeem what is beyond our ability to redeem. To fix all the darkness we failed to turn into light. It might be out of our reach, but nothing is out of Yours HaShem.