Sarah, have you or your physician looked into Seasonal Affective Disorder? It’s a mood disorder subset in which people who have normal mental health throughout most of the year exhibit depressive symptoms at the same time each year, most commonly in winter. Common symptoms include sleeping too much, having little to no energy, and overeating.
Some people seem to find relief by using a specially-designed lighting array that mimics ‘summer’ sunlight. It might be worth looking into.