Probably worth noting that an individual's need for sleep can change in both the short and long term. There are oodles of studies on how sleep need changes as we age, so I won't even go into that. What I've been noticing the last couple of years is the short term variability. I'll go through weeks or months when sleeping six or seven hours feels great, and more than that actually makes me feel worse. Then I'll go through weeks or months when it has to be eight or nine or I'm half dead. Lather, rinse, repeat.

The key, I guess, is to pay attention to your tiredness levels and be prepared to try different things when the current sleep regime doesn't seem to be working. Instead of saying "X hours is what works for me" learn the signs of being overtired and react dynamically.