Sleeping 12 hours is oversleeping. When I oversleep I have a headache for the first half of the day and feel groggy. It's not good sleep. Oversleeping or undersleeping will cause you to feel tired. Some people are able to function on lower amounts of sleep and they typically have the oversleep "issues" if they sleep the average 8. It depends on the person.

My pdoctor recommends sleeping between 7-9 hours a night. Usually if I'm able to get my sleep on track I usually feel better with that much sleep averaged a night. It helps the mood out a lot, too.

What you're feeling is normal though. I have that problem.

Disrupted sleep does the same thing.