Measurement of the brain chemical serotonin shows that its turnover is lowest in winter, perhaps leading to seasonal depression.
Seasonal depression is common - many people feel less energetic and more moody when the dark nights set in. For some, this develops into a form of clinical depression, known as seasonal affective disorder (SAD). Light therapy, and antidepressants that raise the level of serotonin in the brain, are effective in treating SAD. Serotonin is known to affect mood and lack of this brain chemical is associated with depression.

Researchers in Melbourne, Australia, have been investigating the role of serotonin in SAD. They measured serotonin levels in the blood vessels which lead from the brain - an accurate measure of levels within the brain itself. They find that the turnover of brain serotonin is lowest in the winter. What is more, the rate of production of serotonin is directly related to the duration of sunlight. More research is needed to clarify the complex relationship between serotonin, sunlight and depression. In the meantime, getting enough sunlight - by going outdoors every day, or taking a winter holiday somewhere sunny - may be important in avoiding the winter blues.