Hi! I usually sleep at an average of 8 hours. I get enough rest and always ready to go the following day.
However, when I was still schooling, I probably had 4-6 hours. I had to work on a lot of requirements, study every single subject, and attend to every academic demands of my schooling. Like you, I had dark circles in my eyes, looking a bit tired and stressed up, and had a lesser weight than normal. But my body got accustomed to the number of hours I slept back then. Just the dark circles won't disappear. After schooling, I am able to sleep more number of hours that it gives me a more healthy look. Dark circles around my eyes are gone.
Sometimes, people just have those dark circles if they have those deep set eyes.
All I can say is that at times, you'll need to ease up and take a breather. This would enable your body to rejuvenate. If 6 hours of sleep would cause you to be a bit tired with all those dark circles around your eyes, then perhaps, you might need more than 6 hours of sleep.
You can find ways to alleviate your sleeping problems like exercising after work in the pm, taking a hot shower before going to bed, reading a book before sleeping, and a lot more.