In the first study of its kind, the team at the Centre for Addiction and Mental Health (CAMH) in Toronto used positron emission tomography (PET) to determine that brain levels of serotonin transporter are significantly higher in fall/winter than in spring/summer.

Serotonin is involved in regulating emotional functions such as mood and energy levels, and physical functions such as eating and energy balance.

Serotonin transporters remove serotonin, so the findings suggest there is more serotonin removal in the fall/winter than in the spring/summer, said the researchers. They also found that higher serotonin transporter binding (the process that removes serotonin) values occurred at times when there is less sunlight.

The study -- the first to identify seasonal differences in brain levels of serotonin transporter levels -- may help improve understanding of seasonal mood changes in healthy people, vulnerability to seasonal affective mood disorders, and the relationship of light exposure to mood.

This is "an important lead in understanding how season changes serotonin levels. This offers an explanation for why some healthy people experience low mood and energy in the winter, and why there is a regular re-occurrence of depressive episodes in fall and winter in some vulnerable individuals. The next steps will be to understand what causes this change and how to interfere with it," study author Dr. Jeffrey Meyer said in a CAMH news release.

"Over the following years, we intend to determine the specifics of the environment [such as light exposure] that influence serotonin transporter levels so as to determine what is the optimal environment to prevent illness. In the future, it may be that just like we have lifestyle recommendations to prevent heart disease, we will have lifestyle recommendations to prevent major depressive disorder," Meyer said.

More information

The American Academy of Family Physicians has more about seasonal affective disorder .