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As we enter into autumn and winter the nights draw in and daylight hours become less. While some people love the early dark nights as it means relaxing, cosy nights in front of the fire, other people dread waking up in the dark and coming home in the dark. They will spend most of their waking hours under artificial light and, unfortunately, this can lead to symptoms of depression- something no-one wants to experience! It is estimated that approximately 4-6 % of Americans suffer from depression in winter (figures are unknown for the UK) and statistics are higher for people who experience symptoms of S.A.D (Seasonal Affective Disorder) throughout this seasonal period. 
Studies carried out on sufferers of S.A.D, which is known as the winter depression due to symptoms tending to become more severe during the winter months,  have concluded that one of the contributory factors resulting in depression are lower levels of vitamin D and this can be linked partly to a lack of sunlight. This is consistent with the lack of sunlight many people experience during December, January and February, and these are the months when many sufferers find their symptoms deteriorate. However, it isn't just people who live with Seasonal Affective Disorder who lack in vitamin D. Lower levels of vitamin D have also been shown to play a role in diagnoses including all types of depression, psychosis and schizophrenia. A study carried out in 2014 found that people who had attempted to commit suicide had lower levels of vitamin D compared to other subjects in the study. 
Grudet et al. compared vitamin D levels in 59 suicide attempters, 17 non-suicidal depressed subjects and 14 healthy controls.  As well as results showing the suicide attempters had lower levels of vitamin D, they also found that there was a significant association between vitamin D and pro-inflammatory cytokines (molecules that worsen disease) which result in inflammation. Brundin et al. explain that over previous years, inflammation has been found to be a contributing factor in people experiencing depression and suicidal feelings. 
Inflammation can be caused by stress, infection, trauma etc. and vitamin D is known to help boost the immune system which in turn could help to reduce the inflammation. In study subjects with lower vitamin D levels there were higher inflammatory markers in their blood suggesting that low vitamin D levels may cause inflammation. 
As we already know, people suffer from depression and suicidal thoughts/feelings all year round, not only during the winter months, and as yet it isn't clear whether low vitamin D levels actually cause depression or whether they come about due to depression. One of the many symptoms of depression for some people is their inability to leave their home and so, in cases like those, it is possible that the depression has led to low vitamin D levels due to lack of sunlight. However, there is a strong connection between low vitamin D levels and inflammation, therefore further studies are needed in this area to achieve a greater understanding and identify the ways forwards when it comes to helping sufferers in the future. Many physicians are now prescribing vitamin D supplements to patients suffering from depression but as yet there has not been enough research carried out to determine how successful the supplements have been. It is promising, however, that studies have highlighted areas of concern when it comes to depression and suicidal tendencies and hopefully as more research is undertaken, the most effective treatments will soon be found.