people are very bad at evaluating how they are affected by not getting enough sleep

Average people, maybe, but my sleep hours tend to vary and by now I have a pretty good idea how much sleep will leave me a zombie, how much will make me OK but not really in the top form, and how much is enough so that any more is just lazying in bed :-)

It's easy to self-experiment with sleep.

By the way, keep in mind that there is seasonal variation and weather effects.