SAD is thought to be caused by a biochemical imbalance occurring in the hypothalamus when daylight hours are reduced and there is a lack of sunlight. Lack of light causes an increase in the production of Melatonin (which induces sleep) and a reduction in Serotonin (the cause of depression).  

It is evident throughout the northern and southern hemispheres, however is extremely rare in people living within 30 degrees of the Equator, due to the long daylight hours and constant bright light. 

SAD can be a disabling illness for some suffers, preventing them from functioning without medical treatment.  However, a SAD sufferer may experience SAD in a milder form and experience discomfort but not disablement in their daily lives.  This is medically termed Subsyndromal SAD or more commonly 'winter blues'.

Symptoms:

Medically there seems to be an acknowledgement that a diagnosis of SAD can be effectively diagnosed after three consecutive winters where symptoms have presented.  

Symptoms usually present regularly each winter, anytime between September and November and continue until March or April.

Symptoms include:

• Fatigue, an inability to carry out normal daily tasks and daily routines. (lethargy).
• Changes in sleep patterns: desire to sleep more or to excess, difficulties in staying awake or disturbed sleep patterns with early wakening.
• Carbohydrate and sweet food cravings – often which result in weight gain.
• Anxiety, often presenting as feeling tense and being unable to tolerate daily stresses.
• Low moods, feeling loss, guilt, misery, despair and hopelessness.  Some may feel apathy and as though their feelings are numb or have been lost.  Some may experience extremes of mood, possibly even hypomania (over activity) for short periods in autumn / spring.  (Depression and anxiety may be mild or non existent in those suffering Subsyndromal SAD).
• Desire to avoid social and or intimate contact.
• Weakened immune system, therefore more vulnerable to infections / illness.

SAD symptoms clear in spring, either suddenly or gradually usually depending on the intensity of sunlight occurring through spring and early summer.