The average human being needs at least 8 hours of sleep. If you over exert yourself, or miss sleeping one night, you will most likely sleep longer the next night. It's not bad to sleep for 10 to 11 hours, it would be bad if you were sleeping less than 8 though. If you sleep 10 to 11 hours every night and always feel really tired, it means your body is over-resting, so you should set an alarm clock for like 8 am (assuming you go to sleep at midnight) so that you only get 8 hours of sleep. See how you feel after that.