Sudden 'Blue Monday' blues? This is very unlikely, and here is why. Blue Monday is a date of purely commercial nature, invented to encourage holidays sales by a now defunct holiday company and TV channel Sky Travel. Its existence has not been scientifically proven. A large study conducted in the Netherlands even concluded that there was no difference in peoples' moods from season to season, which makes a sudden deep low of one January Monday even more unlikely. Other research showed that the seasonal component may lead to existing depression or other mental health conditions worsening.

General winter blues, however, is not a myth. It is a condition called Seasonal Affective Disorder or SAD - a form of depression that can be quite overwhelming for some people. It is not very widespread and affects around 5-10% of the population. SAD is more common in the Northern Hemisphere where the body clock is disrupted by the lack of daylight in winter. It is also more prominent in women and young adults.

SAD is caused by a lack of sunlight, which leads to a lower level of serotonin a hormone responsible for mood and appetite , a higher level of melatonin a hormone that promotes sleep , and a lower level of vitamin D which some studies found to be correlated with depression.

SAD is not a standalone diagnosis and is missing from some standard diagnostic manuals used around the world. One of them - ICD-10 - only mentions 'seasonal depression'. SAD symptoms are:

• Fatigue
• Difficulty concentrating
• Lack of interest in various activities and feeling hopeless
• Irritability
• Lethargy and general tiredness
• Lower libido
• Feeling low and unhappy
• Weight gain
• Hypersomnia sleeping too much
• Carbohydrates craving