As the long days of summer fade away with the warm weather, many of us start to feel blue. But, that sinking feeling may not be just a normal reaction to the approaching chill of winter - for millions, it's a form of major depression called seasonal affective disorder, or SAD.

Seasonal affective disorder affects about 10 million Americans, and health officials estimate that another 25 million suffer from a mild form of SAD called the winter blues.

In the majority of cases seasonal affective disorder strikes between fall and winter, with symptoms usually beginning in October and lasting until March or April. However, symptoms peak in December, January and February.

Seasonal affective disorder doesn't typically affect people under 20 years old, and about 75 to 80 percent of people who suffer from seasonal affective disorder are women. To be diagnosed with SAD you must have had the symptoms for at least three consecutive autumns or winters, and the symptoms must subside in summer.

Some of the symptoms of seasonal affective disorder are similar to those people with major depression experience. These include:

  • depressed mood
  • hopelessness
  • oversleeping
  • lethargy or lack of energy
  • overeating and weight gain
  • focus or memory problems
  • feelings of guilt
  • suicidal thoughts
  • lack of interest in activities or social interaction

One of the key characteristics of this form of depression is an intense craving for carbohydrates or sweets. Symptoms also aren't related to any life event, such as job loss or loss of a loved one.

What Causes Seasonal Affective Disorder?

Doctors aren't entirely sure, but the main theory is that a lack of sunlight is to blame.  According to the National Institute of Mental Health (NIMH), the long days of darkness in fall and winter affect melatonin secretion, which is controlled by the pineal gland in the brain.

Melatonin is responsible for the sleep-wake cycle and other daily rhythms in the body, states the NIMH. When this hormone's production is affected in autumn and winter, it throws daily body rhythms out of sync and disrupts chemicals in the body that influence your mood, such as serotonin.

The Best Treatments for Seasonal Affective Disorder

The go-to treatment for SAD is light therapy, which alters the biochemistry in the brain by regulating the neurotransmitters serotonin and melatonin, explains NIMH. Thirty minutes of early morning light exposure - to the eyes, not the skin - helps to improve your mood and suppress seasonal affective disorder symptoms.

For the best results, the Mayo Clinic recommends choosing a light therapy box that:

  • is specifically for treating SAD (not all are);
  • has a 10,000 lux intensity even at two feet away;
  • has a UV-light filter to protect your eyes and skin;
  • is LED instead of fluorescent or incandescent;
  • shines the light from above your eyes not directly at them;
  • has a dawn simulator;
  • is portable.

However, light therapy does pose some side effects - usually in the short term - such as nausea, headaches and vision problems. Speak to your doctor about the correct way to use this breakthrough treatment.

Other strategies that can help you survive seasonal affective disorder this fall and winter include:

  • Taking a melatonin supplement - try taking it at midday or in the morning to see which schedule gives you the most relief;

  • Taking a vitamin or mineral supplement (under medical supervision) such as a vitamin B complex, vitamin D, or iron;

  • Spending 10 minutes outside at midday to soak up some rays of the sun and boost the production of vitamin D, which increases serotonin levels in the brain;

  • Using a light therapy box at work to fight seasonal affective disorder if you don't have time to do it at home before the morning rush.