I just don't buy it. The 8 hour number has been purported to be the necessary and optimal amount of sleep for most humans, even for the vast majority, and yet the majority seem to do well on an average of something like 6-7.5 hours.
I used to do well on an average of 7-7.5 hours, and now I'm doing even better on 6-7 after properly supplementing my body with vitamins (I simply began taking vitamin D for a month).
Even when I used to sleep for an average of 7.25 hours, it was rare for me to sleep for 8. I just can't believe that it was necessary for me to sleep for that long on all those nights, except for those that I did.
Your body will simply tell you if you're undersleeping. I don't even know if it's even physically possible for most people to consistently sleep for 8 hours once they're an adult.
Now, you might think that what I'm addressing is just statements of the past: "Everyone is recommending 7-9 hours now, OP. The 8 hour myth is dead."
However, I just read some post somewhere from 2 years ago from someone who put out "Most adults need 8-9 hours to function optimally", which gained a considerable amount of traction, and it's probably because they included that they're a "sleep scientist" and that "This is supported by a pretty robust body of research". But every study on sleep concludes something different anyway, so it just doesn't help at all. (Idea: Just listen to your own body for once instead of inconclusive research.)
All that comments like these do is make people who are sleeping any less than 8 hours worry about their health when it's usually not necessary (because it's usually not an issue), and places a huge feeling of inadequacy on that person who thinks that they're dying earlier because they sleep for 6 hours most nights, or 6.5 hours, or 7 hours, and not 8.