Individuals susceptible to seasonal affective disorder (sad) may be prescribed to seek exposure to bright light for a certain amount of time every day or take selective serotonin reuptake inhibitors (ssri). the reason behind the treatment is to

Increase the neurotransmitter serotonin.

Explanation:

    Seasonal affective disorder (SAD) is a type of depression that is related to seasonal changes. It always happens at the same time of the year: it starts in the fall, extending into the winter.  This type of depression can also occur in spring and summer, although it is less common.

    There is not yet an explicit cause for this disease, but it is believed to have a correlation with the distance from the Equator line because of the light time. Thus, when the sun's orbit becomes lower and the hours of the day shorter, the incidence of seasonal depression is higher.

 Light influences the biological clock. So when there is a low light, there is a breakdown in the production of serotonin - a brain neurotransmitter known as the happiness hormone that affects mood and its related to depression. And the less serotonin, the greater the tendency to sadness and depression. In addition, light also influences sleep-associated melatonin. As winter nights get longer, levels of this hormone increase, which results in fatigue and lack of energy, both symptoms of seasonal depression.

 The treatment for this disorder is the use of lights and SSRI (selective serotonin reuptake inhibitors), which are a class of drugs used in treatments of depression, anxiety and personality disorders. This drugs increase the level of serotonin.