Researchers studied nearly 90 volunteers' brains from 1999 to 2003 and categorised brain images according to season. Brains scanned during the winter and autumn clearly contained the highest levels of a serotonin transport protein which decreases serotonin levels. Brains scanned in the summer and spring had the lowest levels of the protein.
The researchers also noted the weather at the time images were taken and found protein levels were highest when the sun shone the least.
The relationship between serotonin and SAD has been studied previously, however results have been conflicting.
Up to four percent of Finland's population suffers from SAD in the winter. Some 10 to 20 percent suffer from milder symptoms. Many find relief from light therapy.