MELBOURNE, AUSTRALIA. Serotonin is the body's main "mood hormone" with a low level being strongly associated with depression. The depressive effect of low serotonin levels can be somewhat counteracted by the use of selective serotonin reuptake inhibitors (SSRIs) such as Prozac and Paxil. Seasonal affective disorder (SAD) is a highly prevalent form of depression affecting many people during the dark winter months. Analysis of the blood and cerebrospinal fluid of SAD patients has, however, failed to confirm the presence of abnormally low serotonin levels.
Australian researchers now report the results of an experiment, which, in my opinion, is one of the most elegant ones I have come across in a long time. The experiment was designed to determine whether or not brain serotonin levels are affected by sunlight exposure. It involved 101 healthy men, aged 18 to 70 years, with no history of major illness (including depression and SAD) and no current use of medications. Each participant was tested once during the one-year long experiment. On the morning of the test the participants had catheters inserted in the arteries and veins supplying and draining the brain of blood (as close to the brain as possible). Blood samples were drawn simultaneously from the two catheters and analyzed for the level of the neurotransmitters serotonin, norepinephrine and dopamine. The fact that both the supply and the return of blood to the brain were analyzed simultaneously enabled the researchers to determine exactly how much serotonin was produced in the brain under different weather conditions.
The researchers compared brain production of serotonin with mean atmospheric pressure, total rainfall,
highest and lowest temperatures, and hours of bright sunlight during the day. Atmospheric pressure,
temperature and rainfall were not correlated with serotonin production; however, hours of bright sunlight
were, and to quite an astonishing degree. Participants who were tested on a bright day produced eight
times more serotonin in their brain than did people who were tested on a dull day (395 pmol/min versus 49
pmol/min on average). Levels of norepinephrine and dopamine were not affected by any atmospheric
conditions including sunlight. Brain serotonin production was, not surprisingly, highly correlated with the
seasons with levels being seven times higher during the summer than during the winter. The researchers
also observed that the effect of sunlight on brain serotonin production was immediate. Serotonin levels did
not correlate at all with the degree of sunlight experienced on the day prior to the test, but only with sunlight
level on the day of the test. The researchers conclude that the amount of sunlight exposure strongly affects
brain serotonergic activity and thus underlies mood seasonality and SAD.
Editor's comment: There is certainly a great deal of evidence that exposure to bright light first thing in the morning can help alleviate SAD. This, however, as far as I know, is the first study that has actually provided the scientific mechanism for the beneficial effects of strong light. If I were suffering from SAD I would certainly consider investing in a properly designed light box and use it every morning during the dark fall and winter months.