Seasonal Affective Disorder (SAD) is a form of depression triggered at a particular time of year or by a particular season. The significant changes in daylight hours, temperature and weather affect a SAD sufferers mood and energy levels. Symptoms most commonly arise in the cold, dark winter months then disappear come spring as the days grow longer and brighter again.
Seasonal Affective Disorder Symptoms
There are many different symptoms of SAD - you may experience some or all, they include; depression (the 'winter blues'), irritability, anxiety, panic attacks, sleep problems, increased susceptibility to illness, over-eating and a lack of energy to carry out your normal daily routine.
Causes of SAD
A study found that people living in Northern European countries who suffered from SAD showed significant seasonal differences in the way they regulated the neurotransmitter serotonin.  Serotonin (5-HT) is the chemical considered responsible for retaining mood balance, a deficit of serotonin is believed to cause depression. During winter months when there is a decrease in sunlight, patients with seasonal affective disorder experience higher levels of activity in the serotonin transporter protein SERT. An increase in SERT acts to diminish serotonin levels and triggers an onset of symptoms of depression. Those who do not suffer with SAD do not experience an increase in SERT activity, so their active serotonin levels remain high throughout winter. 
Treatment for SAD
If you experience seasonal depression there are a few treatments you could consider:
1. Load up on Vitamin D
As the shorter, darker winter days approach make sure you stock up on vitamin D. Studies have shown there is a link between vitamin D deficiencies and depression and anxiety. 
2. Light therapy
You can combat the symptoms of SAD by providing your body with sunlight at the correct times and in sufficient quantities using an artificial sun lamp. A lack of morning light disrupts our bodies’ circadian rhythms making us feel sluggish and low in energy. On an evening, staying awake long after darkness also causes lack of or disrupted sleep and mood swings.
3. Take 5-HTP
5-HTP is a natural solution to boost your serotonin levels and instantly improve your mood and other seasonal affective disorder symptoms. As a precursor to serotonin, once 5-HTP is ingested and crosses the blood-brain barrier, it interacts with an enzyme that strips off a piece of the molecule resulting in 5-HT or serotonin.