Throughout the years there had been much anecdotal evidence of people who live in northern climates developing what is known as 'cabin fever'. Cabin fever has often been described as a feeling of being depressed caused by being cooped up in a cabin, home, or other enclosed living space for long periods of time.
The reason for being cooped indoors and limited from outside activities can be potentially caused by many factors including sickness, weather, darkness, and the like. However, the reason for the depression was almost always considered to be the confinement itself.
It was not until relatively recently that researchers validated the existence of cabin fever as a real phenomenon. They also theorized that its cause was not the confinement of living in a small space but, instead, was due to a lack of sunlight.
The research into this phenomenon was triggered by Dr. Norman E. Rosenthal, a psychiatrist at the Bethesda National Institute of Mental Health. In his practice, he made the observation that many of his patients who suffered from depression, mysteriously seemed to recover from it as the spring and summer months arrived. He further noticed that many of his patients who visited warmer climates in the Winter were less depressed than they were at home. That is, until they returned home and begin to experience higher levels of depression again. He dubbed this affect, seasonal affective disorder, or SAD for short.
This recognition, prompted Dr. Rosenthal to begin a study into why this was so. And as his research neared its conclusion, it became obvious that what was causing the depression was a lack of light. The research dramatically showed the extent to which light affects a person's mood. It seems that, among people who are prone to depression, the more time they spend in sunlight, the less depressed they are.
The working theory is that what causes this shift in the person's attitude is a hormone called melatonin. The body naturally produces melatonin. And, psychiatrists know that an excess of melatonin in the body can cause depression in a person. It is now recognized that sunlight can inhibit the production of this hormone.
It is also believed that sunlight increases the amount of the neurotransmitter serotonin in the brain. And if the levels of serotonin is high enough, it can offset the negative effects of melatonin. The natural conclusion is that, if you are prone to depression, spending more time in the sun will improve your outlook significantly.