Why when I sleep 8+ hours I feel tired throughout the day, when I sleep 6 or less I could immediately start my day?

When I sleep 8 or more hours it feels like I'm tired throughout the whole day and have trouble getting our of bed. However, when I sleep 6 or less, I could usually jump out of bed and start my day on a good note.

Is there any reason to this? Sorry for the wording, I really don't know how to phrase this correctly.