The twentieth century saw a lot of poorly-informed scientists second-guessing human body mechanims that have been keeping us alive for millions upon millions of years, on the basis of "studies" that constituted tiny, tiny, tiny, tiny samplings of a horrifically n-dimensional parameter space. I don't think the twentieth century scientists came out of it looking too good even now in 2015 and I expect them to continue to look worse as we learn more. This seems as likely a place as any for them to end up having been wrong.

If you look at the studies most people are sleeping between seven and nine hours, which is what the little research we have done on sleep says we need. The "myth" is that, as opposed to what the CDC says, the number of people sleeping less than the recommended hours hasn't increased. Thus the myth that most of us are not sleeping enough.

In addition, when we go to sleep matters, and this can vary, depending on the person.

Finally, the amount of light, noise, the mattress, pillow, etc. all matter. Don't cheap out, but the most expensive mattress is not guaranteed to be the best. Don't sleep on a couch or an uncomfortable mattress.