Is a mood disorder that is caused by the body's reaction to low levels of light present in the winter months

I believe that the seasonal Affective disorder is a mood disorder that is caused by the body's reaction to low levels of light present in the winter months. People find they are only depressed at certain times of the year particularly winter months and goes away with onset of spring and summer. In this disorder people who have normal health through out most of the year exhibit depressive symptoms at the same time of the year especially in the winter. 

Sorry i dont under stand can you rephase that