I was under the impression that 7.5 hours was optimal because it falls within the recommended 7-9 hours and occurs at the end of a sleep cycle, which should help you feel more refreshed and alert upon waking.
Just keep in mind that nothing is exact. That "1.5 hours" is approximate, and will vary between people, and will probably vary from night to night depending on all sorts of things you'd never think of, and maybe between the start and end of the night. Plus I'm pretty sure switching from "awake" to "asleep" isn't instantaneous, so you have to count that time as well.
Yup, sleep cycles are 1.5 hours each and most humans need 5 of them. As long as you don't break out of a sleep cycle before the 1.5 hours is completed you tend to feel pretty awake when you do wake up, even if it's only been 3 hours.
