Summer is the best time to optimize your UVB exposure which gives you vitamin D. But year round sun exposure gives you endorphins and nitric oxide, all essential to overall wellness and better moods.
A fun history of sunshine.
Sunlight therapy was also used in the UK on children to prevent and treat rickets, a condition which affects bone development. Today it is known that as well as spending some time in sunlight, rickets can easily be prevented by eating a diet that includes vitamin D and calcium. But in the 1930s, many schools got sunlamps, and some taught classes outside. Some animals ended up receiving similar treatment.