But those lower serotonin levels bounce back to normal if their seasonal depression is treated -- and in summer.
Researchers say the finding could lead to improved treatments for SAD, which is a form of seasonal depression that worsens in winter and improves in summer. Symptoms include weight gain, increased need for sleep, irritability, and inability to concentrate.
SAD Tied to Serotonin Levels
In this study, researchers at the Medical University of Vienna looked at how the brain removes serotonin through the serotonin transporter and compared the rate of removal in 73 people with untreated seasonal affective disorder and 70 healthy people.
The results, published in Neuropsychopharmacology, showed serotonin was removed from the brain at a faster rate among those with seasonal depression, causing serotonin levels to drop below normal. But serotonin removal rates returned to normal with treatment and during the summer months.
Researchers say the results could help identify people at risk for seasonal depression and more effective treatments.
Currently available treatments for seasonal affective disorder include increased exposure to light sources, such as natural sunlight or a light box, and antidepressants.