I was reading a take-down post about a book called Why We Sleep. The details of this one aside, I think pretty much all of these sorts of pop-science/psychology books are unreliable, especially those making broad and startling claims (like “Getting less than six hours of sleep per night doubles your risk of cancer!”).
To be easily consumable by—and therefore popular with—a lay audience, they have to over-simplify and brush off the uncertainty that underlies any hypothetical claim or research data.
“Sleep is important and we should probably get more of it” seems like a reasonable (and appealing!) notion to me, but stating that way doesn’t sell books, I guess.