There is actually data showing that people who sleep too much have a shorter lifespan. I am sure that is a selection bias though - people with poor health and chronic illnesses tend to sleep more. When my autoimmune arthritis was at it's worst I could easily sleep for 17 hours per day. I can't remember where the cut-off was though for health- maybe at 10 hours? I'll see if I can find the paper.
I can feel pretty crappy if I get too little or too much sleep. Do you change your bedtime at all in the winter months? Or get less exercise?
What about vitamin D3? If you are not getting sunlight during the winter you may have a D deficiency which will cause fatigue.