Why Some People Experience SAD in the Winter and Others are Fine

There may be a reason why some people experience the winter blues while others get through the season just fine. Scientists have found that people with Seasonal Affective Disorder (SAD) show significant seasonal differences in the way they regulate the neurotransmitter serotonin in comparison to the majority of the population.

About one in six people suffer from SAD at Northern European latitudes during the winter. As daylight levels drop in autumn, people suffer from symptoms of depression.

In order to learn a bit more about this condition, the scientists scanned 11 SAD patients and 23 healthy individuals using positron Emission Tomography. This showed that there was a significant summer to winter difference in the levels of serotonin transporter (SERT) protein. SAD patients showed higher levels of SERT in the winter months, which corresponded to a greater removal of serotonin in the winter.

"We believe that we have found the dial the brain turns when it has to adjust serotonin to the changing seasons," said Brenda Mc Mahon, the lead researcher, in a news release. "The serotonin transporter (SERT) carries serotonin back into the nerve cells where it is not active, so the higher the SERT activity the lower the activity of serotonin. Sunlight keeps this setting naturally low, but when the nights grow longer during the autumn, the SERT levels increase, resulting in diminishing active serotonin levels. Many individuals are not really affected by SAD, and we have found that these people don't have this increase in SERT activity, so their active serotonin levels remain high throughout the winter."

In fact, the scientists found that SAD patients had an average 5 percent higher SERT level in the winter compared to the summer. Healthy patients, in contrast, showed no significant change on average.

"SERT fluctuations associated with SAD have been seen in previous studies, but this is the first study to follow patients through summer and winter comparisons," said Siegfried Kasper, commenting for the ECNP. "It seems to offer confirmation that SERT is associated with SAD."