Silmarillion Writers' Guild

The survival of life on Middle-earth in the absence of the Sun is one of those things that has to be hand-waved away. Yavanna did something to plant metabolism. And there was a major crisis of adaption when the Sun appeared on the scene, but everyone was too busy fighting Morgoth to document it properly at the time; or possibly they did, but the documentation was destroyed when the kingdoms of Beleriand fell, and the people who remembered it afterwards were either dead gone over Sea.