Many people report feeling blue during the dark, cold winter months. This dip in mood is thought to be caused by lowered levels of serotonin, a chemical in the brain. Serotonin levels are low in people with depression and, a new study found, also in healthy people during the winter.

Certain people will suffer from seasonal affective disorder (SAD), depression that occurs during the fall and winter months, due to this drop in serotonin levels.

In the study, blood samples were taken from blood vessels leading to the brain in 101 healthy men at several different times over a one-year period. Results indicated that the activity of serotonin-containing neurons was lowest during the fall and winter and highest in spring and summer, when sunlight is most plentiful.

A technique known as phototherapy, in which patients are exposed to bright light, is used to treat those with SAD. Since serotonin levels rise in the brain on days with a lot of sunlight, bright light may boost mood by activating neurons in the brain that contain serotonin, leading to increased levels of the chemical in the brain, researchers say. In the study, serotonin neuron activity was higher on brighter days than darker days, even within the same season. This suggests that levels of serotonin in the brain were directly related to how much sunlight was available on the day the sample was taken.

Though this study involved men, researchers say that women likely experience a drop in serotonin levels during the winter as well, though the actual amount may differ according to gender. Additionally, researchers note that results point to a biological reason for SAD, which may help to displace some of the skepticism regarding this, and other, mental illnesses.

The Lancet December 7, 2002;360:1840-1842