In ancient Greece and Rome, the works of Plato, Aristotle, Virgil and others may have been composed at night, with very little illumination. Monks in the Middle Ages would also have worked all year round and such works as Beowulf or the Anglo Saxon Chronicle would never have been written if everyone in the scriptorium had SAD. 

Did Shakespeare only write in the spring and summer? Candlelight was his only option if he chose to write his plays and sonnets after the sun had set. The same applied to Samuel Pepys, who we can assume wrote at least a portion of his diary entries in the evening, as he recorded his reflections on the day, in all four seasons. And of course Charles Dickens was a prolific writer for twelve months of each year, with only candlelight, oil or gas lamps to enable him to write.