We know that not sleeping for days, or sleeping significantly less per day than 8h, doesn't cause any serious problems in healthy adults.

Maybe if you do something simple. I experience serious mental impairment on the whole next day even if I sleep 7 hours (in the sense of not being able to get on with reading math or thinking as fast and productive/deep as after good sleep). This isn't about sleepiness, as I can be perfectly alert, just dumber (there's also coffee that helps with sleepiness but not with thought). This is probably too fine a point to have been adequately researched.