Chronic 6hr/night Sleep -> Incompetence
“chronic restriction of sleep to 6 h or less per night produced cognitive performance deficits equivalent to up to 2 nights of total sleep deprivation.” [4hrs/night after 7 days, or 6hrs/night after 14 days, is about the same as skipping a night’s sleep, as far as your competence goes - but not your sleepiness]
Subjective sleepiness doesn’t track impairment. In other words, you can feel the same sorta-tired constantly for a few weeks while your performance keeps getting worse.
The study looks at restricted sleep over a 2 week period. The quantity that predicts impairment best isn’t “sleep debt” with respect to an ideal 8hr/night target, but rather “extra hours awake, cumulative” (so if you steal an extra 2hr/day for 10 days, by the end you’re as impaired as at the end of a 36hr stretch of wakefulness - because normally you’re awake 16hrs). The “sleep debt from 8hr/night”, depending on how you calculate it, at the end of 36hr awake, would probably be considered only (36hr*(8hr/24hr))=12hr, or more optimistically, (16hr awake, should have slept 8hr, but slept 0hr, 16hr awake, should have slept another 4/3 hr, but slept 0hr, awake 4hr, for a total of 36hr awake, 9.333 hr debt).