SAD is also known as Winter depression/Winter blues. Symptoms usually start in the autumn and continues into the winter months, lowering one's  energy and making feel moody. Rarely does it cause depression in the spring or early summer.

Causes

The precise cause of SAD is not known. It can be due to variation in combination of factors like

• Mental health condition
• Genetics
• Age
• Chemical make up of the body

Some of the specific factors are

• Serotonin levels: Reduced sunlight can cause a drop in serotonin levels

• Melatonin levels:Change in season can disrupt the balance of the natural hormone melatonin, which plays a role in sleep patterns and mood.
  

• Variation in Circadian rhythm: Decreased sunlight can disrupt the biological clock and can have an impact on sleeping patterns.

Risk factors:

• More common in females
• More common in people with family history
• People living in regions with decreased sunlight

Symptoms 

• Depressed feeling
• Hopelessness
• Anxiousness
• Loss of interest in activities
• Increased appetite-craving for foods mainly sweets
• Decreased concentration
• Social withdrawal
• Oversleeping
• Lack of energy

Complications

• Suicidal thoughts
• Learning problems
• Problems with work
• Substance abuse

Treatment

Phototherapy(or Light therapy): Light therapy simulates  outdoor light and causes a change in brain chemicals that affect the  mood of a person. Treatment is easy to use and seems to have few side effects. The mechanism of action is not known clearly but is helpful.

Medications: Indicated when the symptoms are severe. Antidepressants are the medications of choice. Some of the medications used are

• Bupropion
• Sertraline
• Paroxetine
• Fluoxetine
• Venlafaxine

Psychotherapy: Is quite helpful in identifying and correcting negative  

Life style changes

• Exercises help in a significant manner to reduce the symptoms and feel better.
• Going out often and exposure to sunlight has beneficial effect.