Well, we now know a lot more about the health effects of lack of sleep than we did in like, the 1970s, which is when he started. And, I imagine if you're otherwise very healthy and "acclimate" to the short sleep schedule, you don't really notice the signals that you're running on a deficit.

I think people also thought that there were wider ranges of acceptable sleep, depending on the person, and it's turning out that's not true.

All of this "no, really, literally everyone needs eight hours of sleep every single night of their life no exceptions or it will actually kill you dead" has only come to light in the past two decades.