Many issues with why you sleep long hours... some mental, some physical, some environmental. Trick is not to work out why you are sleeping such long hours, but work out how to rectify the problem by finding a solution that works for you. There are physical attributes that some may think is depression which cause this, being you are doing nothing physical during the day which produces not energy for your body, so you feel tired because your doing nothing. It becomes a nasty cycle. If you have ever heard or been told, exercise or physical activities promote the body to produce more energy, then that is one very common problem. People think they have no energy, so they then use that as the reason to not exercise, so the body continues producing no chemicals that tell the brain the body has energy to burn, so the body instead says sleep. Very nasty cycle and it looks like depression, but its not depression, its your bodies natural ability to sleep or produce energy.

So to discover the problem you must try various solutions. Go walking, even though your body says sleep, go for a good walk, hour minimum and then assess whether you now have energy to do more or whether you need sleep. If you exercise and are physical for a good few hours and your body still says sleep, then you look towards other problems.... like environmental (your house, living standard, cleanliness, etc) or mental, being depression or other chemical issue.

The most common that can solve 95% of the cases, is lack of daily physical activity with regularity. Routine is a major aspect of this. You must make yourself get in a routine, being you wakeup and get out of bed at the same time, you do not just move from the bed to the lounge, you incorporate structure and physical activity to your day. If you do not, your brain gets bored basically speaking and tells the body to sleep.