What dark evenings do to our minds

Every autumn, the clocks go back and the evenings close in. By late afternoon the light is draining out of the sky, the temperature drops, and something shifts in the mood of the room. Conversations turn to the cold, the dark and the long months ahead.

It is a very human reaction. But what is actually happening in our minds when the days shorten, and how much of it comes from the dark itself, rather than from the story we tell ourselves about it?