According to scientists, seasonal changes in brain chemistry cause winter blues in some people.
Seasonal affective disorder is related to lack of light exposure during short winter days and it often proves quite debilitating in some people.
The study shows that the condition may develop proteins that make mood-regulating chemical serotonin more active in the brain.
The study has been published in Archives of General Psychiatry and it was conducted by the researchers from University of Toronto.
Lack of energy, fatigue, depressed mood, overeating and tendency to sleep longer are often linked to seasonal affective disorder and this condition affects thousands of people in the UK and other European countries every year.
The researchers wrote in the journal that the people who lived in temperate zones often felt happier and energetic on bright and sunny days and during the dark winter days, they experienced decline in mood and energy.
In the study, the researchers made brain scans on 88 volunteers during 1999 and 2003.
The scans were carried to note the activity of a protein that is known as serotonin transporter and works to bind serotonin and clears it as well from the spaces in brain cells.
The researchers noticed that this protein was more active in different areas of the brain during autumn and winter. The researchers concluded that light had a direct effect on the protein activity in brain.