The scentific reason why the sun makes us happy

Psychologist Mark E. Beecher of Brigham Young University conducted a study involving 16,452 adults that analyzed therapeutic distress across 19 weather and pollution variables over a six-year period. He found that seasonal increases in the amount of time spent in the sun were linked to decreased emotional stress—whereas temperature and pollution variables had no effect. 

When it’s wintertime and the days are short and gloomy, many typically experience feelings of depression and helplessness. But when the spring and summer seasons roll around, we often find ourselves feeling more uplifted and eager to start the day. 
             GREAT SUMMER ART
In 1997, in an attempt to bring joy to the miserably light-starved people of Helsinki, psychiatrist Timo Partonen of the National Public Health Institute gave people special lamps producing light that closely matched sunlight. After leaving the lamps on their desks throughout the winter people felt happier, less hostile and more alert.
Light also triggers changes in the brain that make us feel more cheery. Evidence is emerging that light pushes up levels of serotonin and noradrenaline, two key feel-good chemicals.

The web connecting sunlight, melatonin and serotonin goes something like this: When the sun comes up again, and sunlight hits the optic nerve, some of that light is sent to the gland in the brain in charge of melatonin. In response, melatonin secretion decreases. When the sun goes down, the body increases its secretion of melatonin.

At the same time, when the body perceives sunlight, serotonin levels increase. And the more sunlight the human body is exposed to, the more serotonin the brain produces [source: ScienceBlog]. So in effect, melatonin and serotonin have an inverse-proportional relationship that is guided by the body's perception of sunlight. The overall effect is "downtime" at night and "uptime" during the day.        
  GREAT SUMMER ART
There's another factor involved in sunlight's affect on mood, though: vitamin D. The body actually creates its supply of vitamin D from the sun's ultraviolet rays hitting the skin, and high levels of vitamin help the body maintain high levels of serotonin [source: Collinge].