Sleeping less than 6 hours impairs you as much as no sleep

Research by sleep scientist Matthew Walker and David Dinges found that sleeping 6 hours a night for two weeks produces cognitive impairment equivalent to 24 hours of total sleep deprivation — but the subjects didn't feel impaired. People become bad judges of their own sleepiness after chronic restriction, meaning millions of people drive, work, and make decisions severely impaired without knowing it. No amount of caffeine fully compensates.

Source: Van Dongen, H.P. et al., 'The cumulative cost of additional wakefulness', Sleep (2003)