Researchers may have discovered the biological reason why winter leaves millions of Americans feeling tired, irritable, and depressed.

Some 4 to 6 percent of Americans suffer from seasonal affective disorder (SAD), a type of  depression that occurs during the same time each year.

Biologists have known that variations in the amount of sunlight a person receives and its effect on her or his circadian clock (our internal mechanism that responds to changes in light) play a role in the disorder. However, they did not know the underlying neurobiological process responsible.

Now a team from Vanderbilt University, using mice, has discovered that mood changes characteristic of SAD are controlled by the small region in the mid-brain, called the dorsal raphe nucleus, which controls serotonin levels.

High levels of the hormone serotonin in the brain are linked to feelings of well-being and happiness. Low levels correlate with depression.

Mice have the same type of region in their brains as humans, so scientists divided them into three groups, each born and raised in an environment that replicated the amount of light found in summer, spring/fall, and winter.

The researchers found that the summer-light-cycle mice exhibited lower levels of depression-like behavior than their spring/fall- or winter-light-cycle counterparts.

They also found the mice in the group born during the summer-light-cycle mice had elevated levels of serotonin and the neurotransmitter norepinephrine, which is known to excite serotonergic neurons.

The study is reported in the journal Current Biology.