Sun

To see sun in dreams can ean many things, based on connections with other symbols or signs. So lets look what it means exactly:
Shining sun, bright day: Good future.
Sun and rainbow: Really good sign.
Sun going down: Problems.
Sun over cloud: You will win over dificulties.

Dream

Lets think why people dreams. Why it happend only when we sleep? How work mechanism which determine when you will dream? Its easy, by physical meaning, there are several phases of sleep, during which is brain in different states. And one of them is called REM (by Rapid Eye Moving) when your eyes moves rapidly, and thats time when you dream something. And also its time when you rest best, when there is dysfunction, and you cant reach REM sleeping it will damage psychic and you can go mad.