Reddit, I just slept for 13 hours. Why does my body so badly want to continue sleeping even after getting more than 8?

Although I slept more than 8 hours, I didn't even wake up until 13 had elapsed. I knew I had gotten plenty of sleep by then, but trying to get up was difficult cause I still felt tired. Why does this happen?

EDIT: Posted to r/AskScience.

Edit for Clarity: I just started a new job a few weeks ago that is graveyard hours. However, I still end up sleeping over 10-11+ hours when I get home. I should also not that when I do eventually wake up, I'm not necessarily exhausted once I'm up and moving around. But waking up can take me up to an hour cause my body feels so tired.

EDIT again!: Thank you so much for all the responses and suggestions, everyone! So far I've got:
-Possibly dehydrated. Drink more water.
-See doctor for possible hypothyroidism, and just make sure everything is working correctly.
-Calculate sleep and set alarms to prevent oversleeping.
-Working a graveyard shift naturally messes up sleep cycle.
-Various tips and tricks.

All the information you've provided is super helpful. And I'll keep trying to reply to all your comments. :)