Our biochronological clock is set to a 24 hour period, which incorporates the aproximate time of the rising and setting sun.
The light from the sun (and only natural light) can create melatonin, which is released during the night. The amount of light recieved during the day affects the amount of melatonin created during the night. However, this is an inverse effect, or in other words, the more sunlight during the day, the less melatonin released during the night, and vice versa.
The more melatonin released into your body, the harder it is for your body to go from a sleep to wake state of being.
In the winter, the days grow shorter (in the northern latitudes moreso), and more melatonin is released during the longer night. This causes the person to go through a drowsy state, become lathargic and ultimalty depressed during the winter.
Because of this increased level of melatonin, some of the population experience an inablility for their internal clocks to reset to the new aproximate times of night and day, thus they become affected by high melatonin and S.A.D.