The main determinant of quality sleep is completing sleep cycles and Rem.
Most times, sleeping for 6 good hours can make you feel better than an 8hours sleep in which u interrupt your last circle.
The length of these cycles varies between individuals,but tend to be 1.5 hours in length.
Also, know that waking up multiple times during the night can interrupt the these cycle leading to poor sleep ,
Finally.your circadian rhythm can dictate when you want to sleep so,going to bed later,but still getting your 6.8 hours of sleep can still be of low quality.
You can discourse your own experience.