Sleep isn't downtime — it's when memory consolidation, metabolic clearance in the brain, hormone regulation, and immune function happen. Short sleep is causally linked to impaired glucose handling, increased appetite (ghrelin up, leptin down), weakened immune response, and worse emotional regulation, with much of this demonstrated in controlled experiments rather than just correlations.
The most dangerous feature is that subjective impairment doesn't track objective impairment. After days of restricted sleep, people's performance keeps degrading while their self-rated sleepiness plateaus — they feel about as bad as they did on day two but perform far worse. This is why so many people believe they've adapted to six hours a night. They haven't adapted; they've just lost the ability to perceive the deficit, which is exactly what makes chronic short sleep so easy to sustain.
Hans Van Dongen and David Dinges at the University of Pennsylvania ran a two-week laboratory study restricting participants to 4, 6, or 8 hours in bed, measuring cognitive performance daily. The 6-hour group — a sleep duration millions consider normal — showed cognitive deficits after two weeks equivalent to going 24 hours without sleep at all, roughly the impairment of being legally drunk. The finding that made the study famous was the dissociation: participants in the 4- and 6-hour groups rated themselves as only slightly sleepy, and their subjective ratings stopped worsening after a few days, even as their objective performance continued to decline in a straight line. They had no idea how impaired they were. Van Dongen's paper, published in SLEEP in 2003, concluded that people chronically restricting sleep are systematically unaware of their accumulating deficits.