I used to wait until nighttime to get all of my work done because I thought I was most productive at night, but found myself dragging/taking naps/having trouble focusing during the day, so I started forcing myself into doing work during the day and turning off the lights by about 1 AM and trying to get up about the same time every morning (between 6:30 and 7:30) and am now much more alert and productive with my time, and definitely not as angry at the world, and also less prone to cry for no reason 🙄 I think consistancy is as important as amount, as is quality. Another thing that really helps for me is getting in regular exercise and not od'ing on caffeine. So I guess 6-8 hrs is my window- less than 6 and I'm sort of fried for the day unless I drink lots of coffee....