What is it?
Seasonal affective disorder, SAD, is a mood disorder in which people who otherwise have normal mental health experience symptoms of depression during the same time each year. Most commonly this time is winter.
Typical symptoms of SAD are those also seen in depression. They include persistent empty or sad feelings, hopelessness, a low self esteem, irritability, sleeping and eating more or less than normal, and a reduced sex drive.
The prevalence of SAD varies a lot by area. In Florida it affects 1.4% of the population, for example, while in Alaska 9.9% of people suffer from it.
What causes it?
Animals struggle to survive in cold and dark environments, which is why many of them have evolved to reduce their levels of activity during winter. Food is hard to find during these months, so it is advantageous to not waste calories in unnecessary movement. Human babies are born 9 months after conception and it is harder for them to survive the first critical months if it’s cold and there’s a lack of resources. It is therefore a fair proposition that SAD is also useful because it prevents people from having kids who are born right before winter kicks in.
People with higher levels of certain personality traits like neuroticism and agreeableness are more prone to SAD than others. It has also been found that the levels of SAD in Iceland and Japan are lower than would be expected given their location, and it has been proposed that the reason for this is the large consumption of fish in these countries.
What happens during it?
The mechanism of SAD isn’t fully understood, but researchers have been able to bring some pieces of the puzzle together. The fact that the prevalence of SAD correlates with how dark it gets in the area during winter, as well as the responsiveness of SAD patients to bright-light therapy, suggests that the symptoms of the disorder are closely linked to light.
Melatonin is a hormone that makes us sleepy. Our brains excrete it in regular patterns – even if we would live our lives in completely dark rooms with no knowledge of when it’s daytime and when it’s nighttime, we would still have a sleep-wake cycle lasting about 24 hours. Our natural cycle isn’t exactly 24 hours long, though, and that’s why our brains use cues like light to help sync the body’s rhythm with the world’s rhythm. Being exposed to bright light makes our melatonin production stop. Some studies have indicated, that the unusually low levels of light during winter “mess” with the internal clocks of some people, and the result (internal clocks are not in sync with society-imposed sleep-wake cycles) plays an important role in the development of SAD.
Serotonin is a neurotransmitter in the brain, which means that the movement of serotonin molecules between neurons is what gets information flowing between them. Changes in serotonin levels are often associated with depressive symptoms. Our nerve cells release the serotonin from inside them to reach the adjacent nerve cell, and when the molecules have done their thing, they are taken back inside the original nerve cell. This is done by a transporter protein called SERT. A recent study found, that while in people without SAD the levels of SERT remain the same throughout the year, in people with SAD the SERT levels are higher during winter. This means that when winter arrives and people prone to SAD start to produce more SERT, their serotonin is increasingly carried back inside cells – where it isn’t active.The decreasing levels could lead to the symptoms of depression.
How can it be treated?
Light therapy can be useful in the treatment of SAD, as it can help with getting the internal clocks in sync with the external world by altering the rhythm of melatonin production. Other than that, SAD should generally be treated in the same way as normal depression. This can include things like cognitive behavioral therapy and the use of antidepressants like SSRIs.
Sources: https://en.wikipedia.org/wiki/Seasonal_affective_disorder, http://www.nhs.uk/Conditions/Seasonal-affective-disorder/Pages/Treatment.aspx, http://onlinelibrary.wiley.com/doi/10.1002/pnp.173/pdf