Anecdotal evidence suggests that the weather can affect our mood, with susceptible individuals developing seasonal affective disorder (SAD) — a depressive disorder associated with increased appetite and hypersomnia that occurs during the winter and wanes in the spring and summer. The mediators involved in these mood swings have been unclear, but in December 7 The Lancet, Gavin Lambert and colleagues at the Baker Heart Research Institute, Melbourne, Australia, show that sunlight level and season influence serotonin turnover in normal human brains (The Lancet, 360:1840-1842, December 7, 2002).

Lambert et al. placed catheters high in the jugular vein to directly sample venous blood from the brain. They took blood samples from 101 healthy men and observed that turnover of serotonin by the brain was lowest in winter (p=0·013). In addition, they showed that the rate of production of serotonin by the brain was directly related to the prevailing duration of bright sunlight (r=0·294, p =0·010), and rose rapidly with increased luminosity.

"Our observations suggest that the prevailing amount of sunlight affects brain serotonergic activity, and thus underlies mood seasonality and seasonal affective disorder, although we do not know whether patients predisposed to affective disorders are affected by environmental factors in the same way as our healthy volunteers", conclude the authors.