What Causes SAD?
When the light passes through the eyes into the brain, serotonin is released. During the fall and winter, there is less daylight than in the spring and summer, which causes a drop in the body's serotonin levels.
Serotonin is an important chemical in the brain known as a neurotransmitter. A neurotransmitter is a molecule in the brain that helps nerve cells to work together. One of the roles serotonin has in the brain is to act as a traffic cop to other neurotransmitters. Without enough serotonin, a wide range of body functions is affected, including mood. Less daylight is a trigger for the body to increase production of a certain hormone - melatonin. The role of melatonin is not clearly understood, but it is thought to help in the sleep process. The body releases it at night, during sleep.
Together, the lack of serotonin (which helps nerve cells cooperate) and the increase in melatonin (which put a body to sleep) cause SAD.