Lihua Sun, a student from China who moved to Finland for graduate studies, noticed a trend in Finland. He began to see that at different points of time, people became moody. His hypothesis was that it had to do with the change of seasons. For his graduate studies in Finland, Sun noticed a change in his moods after his 14 years living there. His study was to take brain scans of mu-opioid of healthy volunteers to see how much of the chemical was visible in the scan. He noticed that the mu-opioid was more visible during the hours of sunlight. In contrast, the mu-opioid was least abundant during hours of no sunlight.
What you can conclude from this is that the spikes in mu-opioid cause changes in mood. Sun noted that studies have shown that suicides have peaked during hours of long sunlight and long darkness. This shows that mu-opioid needs to be exposed to humans in moderation. The paradigm of prolonged darkness causes depression can now be stated as false, since depression can be noticed from prolonged sunlight as well. We must be careful about our moods during the Summer and winter, due to the long lasting sun and moon.