Do you have to wake up at the exact same time? I wake up with the alarm only if I go to bed too late, otherwise I assume I have to sleep for 10 hours, go to be

Do you have to wake up at the exact same time? I wake up with the alarm only if I go to bed too late, otherwise I assume I have to sleep for 10 hours, go to bed and my body naturally wakes up between 830 and 930 hours of sleep. Very resting. Also if I gwt 2 days of 9 hours, the next day usually I end up sleeping 8 and still feel rested, so it seems the body naturally adjusts