Natural sunlight is a good healer for many people with psoriasis, but there are some who do not respond and some who are made worse. Sunlight consists of several different types of light across a spectrum ranging from infra-red, through visible light, to ultraviolet (UV). It is the ultra-violet part that can help in treating psoriasis.
The skin cells contain specialised molecules (called chromophores) that are capable of absorbing the energy from UV light and then releasing it to power chemical reactions that affect the function of the cells. In psoriasis, this can result in the cells not multiplying so rapidly and behaving more like normal skin. A lot has been said about the Dead Sea and its benefits, and I understand that some countries such as Germany will help towards the cost of treatment there.