Sunlight helps to regulate the hormones serotonin and melatonin, it has also been shown to affect brain blood flow. Thus, sunlight may not only affect mood directly, but also via its effect on cognition, because cognitive impairment can underlie a depressive episode.
A lack of vitamin D, which is synthetized in the presence of sunlight, can lead to an increase in low mood, fatigue, and depressive episodes. On the other hand, it is also important to keep in mind that too much sunlight can also have detrimental health effects.
Time Magazine has followed the trail of the benefits of sunlight in “Why Sunlight Is So Good For You“. HealthLine provides an overview in “What Are the Benefits of Sunlight?” There is a long list of conditions, psychological and bodily, that have been associated with a lack of sunlight.
A type of depression that is directly related to the seasons is Seasonal Affective Disorder (SAD). People who have this type usually are more inclined towards low mood in the winter months when there is less sunshine. You may want to read further on this topic in the article Seasonal affective disorder (SAD) published by the Mayo Clinic.
There is also some empirical evidence that a lack of sunlight can decrease our cognitive functioning, our ability to think about problems and concentrate on them. One study titled “Effect of sunlight exposure on cognitive function among depressed and non-depressed participants” sought to demonstrate this.
