"The group who got six hours of sleep seemed to be holding their own, until around day 10 of the study."
So they were deprived of 20 hours of sleep (based on the 8 hour control group). The "no sleep" group was deprived of 16 hours of sleep. Both groups were deprived around the same number of hours of sleep. A more enlightening conclusion should be that "sleep debt" exists and is harmful, and we shouldn't feel bad sleeping in on weekends!