Some people suffer from 'winter blues' while others have no issue.

Seasonal Affective Disorder (SAD)
affects people as daylight levels drop in autumn. At Northern European latitudes (for example all of Scandinavia, Glasgow and Moscow) around 1 person in 6 suffers from SAD. 

Psychologists have searched for reasons why. A small longitudinal study concluded that people with Seasonal Affective Disorder show significant seasonal differences in the way they regulate the neurotransmitter serotonin in comparison to the majority of the population. 

The researchers scanned 11 SAD patients and 23 healthy individuals using Positron Emission Tomography; they were able to show significant summer to winter differences in the levels of the serotonin transporter (SERT) protein; SAD patients showed higher levels of SERT in the winter months, corresponding to a greater removal of serotonin in winter.

Serotonin (also known as 5-HT) is a neurotransmitter which affects mood, in fact many anti-depressant drugs, such as SSRIs (Selective Serotonin Reuptake Inhibitors, such as Prozac) work by allowing serotonin to be retained in the synapse where it exerts its effects.

Lead researcher, Brenda Mc Mahon said, "We believe that we have found the dial the brain turns when it has to adjust serotonin to the changing seasons. 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."

The SAD patients had an average 5% higher SERT level in the winter compared to the summer, whereas the healthy participants on average showed no significant change.