Unless there is extend of stress or body injury that needs sleep to continue the recovering process, or other medical situation occurs, a healthy body does NOT require more sleep than 6-7 hours, because it is not about sleep's length but the quality of REM sessions of your sleep. If you happen to to push your sleep over 8.5 hour, you are entering into another REM session (that you don't need) that's why when you wake up in the middle of from it, you feel very tired. But try to wake up precise 6.5 hour after you fell asleep and you will feel very refreshed (given you had interrupted good night sleep)