TUESDAY, Sept. 2 (HealthDay News) -- Fluctuations in the actions of the serotonin transporter, which helps regulate the mood-altering neurotransmitter serotonin, may help explain seasonal affective disorder and related mood changes, researchers say.

In places where the weather changes with the seasons, people commonly feel happier and more energetic when the days are bright and sunny and more depressed and fatigued during the dark of winter. Scientists believe this is related to variations in brain levels of serotonin, a neurotransmitter involved in regulating functions such as mating, feeding, energy balance, and sleep.

In a study published in the September issue of the Archives of General Psychiatry, researchers from the Centre for Addiction and Mental Health and the University of Toronto had 88 healthy people undergo a positron emission tomography (PET) scan to assess serotonin binding potential, which indicates serotonin transporter density. The higher the serotonin binding potential, the less serotonin that is circulating in the brain.

To study seasonal fluctuations of serotonin binding potential, the researchers grouped the PET scans according to the season of the scan -- fall and winter or spring and summer.

The serotonin binding potentials were significantly higher during the fall and winter months than in the spring and summer, indicating that less serotonin circulates in the brain during the darker, colder time of the year. The researchers compared their findings to meterological data and found higher values of serotonin binding potentials during times when there were fewer hours of sunlight each day.

The researchers said that higher serotonin binding potential in the winter may help explain why people report lower mood, lack of energy, fatigue, overeating, and increased sleeping during the darker seasons.

"This offers a possible explanation for the regular reoccurrence of depressive episodes in fall and winter in some vulnerable individuals," the researchers wrote.

More information

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