Doing well on less would mean simply becoming accustomed to sleep deprivation. A study from the University of Pennsylvania looked at subjects limited to eight, six, and four hours of sleep, respectively, for two weeks, in addition to a group that stayed awake for 88 hours. The eight-hour group showed stable mood and functioning. Interestingly, the group that slept four hours performed as poorly on their tasks as the group that didn't sleep for 88 hours, but their mood was on a level with the six-hour group.