Poor mood in winter is linked to lack of light
Scientists have pinpointed seasonal changes in brain chemistry which may cause some people to suffer from the winter blues.
Seasonal affective disorder, which can be debilitating, is linked to lack of light exposure on short winter days.
The latest study suggests this might make proteins which clear the brain of the mood-regulating chemical serotonin more active.
The University of Toronto study appears in Archives of General Psychiatry.
Seasonal affective disorder is linked to lack of energy, fatigue, overeating and a tendancy to sleep longer as well as depressed mood.
The condition affects thousands of people in the UK.
Writing in the journal, the researchers said it was common for people living in temperate zones to feel happier and more energetic on bright and sunny days, with many experiencing a decline in mood and energy during the dark winter season.
The researchers carried out brain scans on 88 volunteers between 1999 and 2003.
The scans were designed to assess the activity of a protein known as a serotonin transporter, which binds to serotonin and clears it from the spaces between brain cells.
The more active the protein, the lower the levels of serotonin in the brain, and the greater the likelihood of a depressed mood.
The researchers found that the protein was significantly more active in all areas of the brain examined by the scans in the autumn and winter.
They believe light may have a direct effect on the activity of the protein.
The researchers wrote: "These findings have important implications for understanding seasonal mood change in healthy individuals, vulnerability to seasonal affective disorder and the relationship of light exposure to mood.
"This offers a possible explanation for the regular reoccurrence of depressive episodes in fall and winter in some vulnerable individuals."
Dr Jonathan Johnston, a lecturer in neuroscience at the University of Surrey, said: "The data show a correlation between a serotonin transporter chemical and hours of sunshine, although how day-length might change transporter activity is not yet known."
Professor Michael Terman, an expert in seasonal affective disorder at Columbia University in New York, said the causes were likely to be complex.
He said the condition might be linked to disturbance caused to the body's natural daily rhythm by the fact that dawn and dusk were closer together in the winter.
He said research had shown that symptoms of winter depression had been reduced by brief exposure to light around dawn.
This suggests that the timing of exposure to light, rather than the simple volume of exposure might be important.