How is S. A. D. Contracted?

Many species change their behaviors during the winter months, some hibernate, but even those that do not still exhibit behavioral changes during the winter.

No specific gene has been shown to cause SAD
many people with this illness report at least one close relative with a psychiatric condition (most frequently a severe depressive disorder or substance abuse)

The neurotransmitter serotonin may not be functioning optimally in many patients with SAD.

The role of hormones, specifically melatonin, and sleep-wake cycles (also called circadian rhythms) during the changing seasons is still being studied in people with SAD.

SAD may be caused by lack of light in winter seasons.
People with SAD produce higher levels of the hormone melatonin in winter, compared with people who do not have the condition.

The pineal gland in the brain produces melatonin when we are exposed to darkness, which causes us to sleep. When it is light, the production stops, and makes us wake up.