This is not true for everyone. We all have different reasons for similar feelings like this. I sleep great but still feel like this. Oversleeping is also a sign of depression.
For me, getting my diet under control and truly understanding what my body needs has helped tremendously (more fiber/protein, less carbs/sugars). Introducing daily probiotics has helped too. It really is amazing how our diets affect our moods.
For some, regular exercise may help. For others, sun lamps help with seasonal depression. Sometimes you need medication. Sometimes weed helps.
Always talk to a professional. Try different things until you figure out what works and what doesn't.
If someone walks into your house and shoots your family, would you want to know why? Would it be important to you to understand their motives?