By Marc Sorenson, EdD, Sunlight Institute
Serotonin is one of the most potent natural mood enhancers produced in the body. As previously mentioned in one of my blogs, men whose levels of serotonin were measured on a very bright day produced eight times more serotonin than those who were measured on a cloudy, dismal day. Serotonin levels were also seven times higher in summer than winter. Serotonin works in concert with melatonin; serotonin keeps keeps us bright and happy during the sunny day and melatonin lets lets us sleep during the dark night. Unfortunately, lack of sunlight in winter may also lead to too much daytime melatonin production, which may lead to aggressive behavior.
A 2015 report from Proceedings of the Royal Academy B, demonstrated that female rats who had the least sunlight exposure per day displayed the most aggression. A similar increase in aggression was not observed in male rats.
An increase in melatonin was given as the reason for the aggression, since when winter approaches, sunlight decreases and melatonin increases. (Of course, serotonin also decreases, as stated in the aforementioned research.) Melatonin increases aggression, according to this study, by acting on the adrenal glands, causing a release of a hormone called DHEA, which has been consistently been linked to aggression in both mammals and birds.
I’m not entirely convinced that that aggression was not caused more by the drop in serotonin than the increase in melatonin. Either way, you boyfriends and husbands should be sure that your lady love gets plenty of sunlight or other form of UV light every day of the year, or you may be in trouble!
 Lambert GW, Reid C, Kaye DM, Jennings GL, Esler MD. Effect of sunlight and season on serotonin turnover in the brain. Lancet. 2002 Dec 7;360(9348):1840-2.
 Proceedings of the Royal Academy B 2015. Reported by Carly St. James, Empire State News. http://www.empirestatenews.net/2015/11/19/a-spike-in-female-aggression-can-be-linked-to-this-seasonal-change/ [accessed November 19, 2015]}