This. When I'm lucky I sleep 6 or 7.5 hours. If I wake up after 6 hours and I can sleep more, I automatically start another cycle and then I sleep pretty well. But if something wakes me up before that time, it ruins my day, because I can't really get back to sleep or maybe I start another cycle and it doesn't finish again. I'm not sure.
I suffer from insomnia, but I have flexible work hours, so I can usually adjust it and that makes those lucky days. However, when I have an early appointment I end up sleeping 4-4.5 hours because I can't fall asleep soon enough. Sometimes it's 4.5 and and don't have much problem, so long as I can sleep well the next night.
Recently I discovered that at least part of my insomnia, the one when I wake up in the middle of the night, is not really that bad, because of this:
Insomnia: Why your brain may sleep without even knowing it