The researchers found on perceptual tasks, performance declined with sleep deprivation. Participants significantly overestimated their performance and confidence. Performance did not significantly suffer over time on the quantitative tests, but sleep-deprived people still consistently overestimated their achievement.
An earlier study, published in 2004 by University of Pennsylvania School of Medicine researchers, also highlighted this phenomenon. Over 14 days, they tested groups of people who received either four, six or eight hours of sleep on several tasks. They found that cognitive performance continuously declined for the four- and six-hour groups, depending on the amount of sleep.
Even for the six-hour group, the cognitive deficits were equivalent to two nights of complete sleep deprivation. Researchers also found that participants underrated sleepiness and were largely unaware of increasing cognitive impairments.
And a French study also found sleep-deprived drivers overestimated their abilities. The study authors summarized in 2003, “The lack of correspondence between reaction time and prospective self-evaluation of performance suggests that self-monitoring in real conditions is poorly reliable.”
Essentially, the mind and body appear to get used to the new baseline of chronic tiredness. This leads to underestimating potential changes in attention, performance and behaviors. So people who have been scraping by on a few hours of sleep a night might not be in as good as shape as they would like to believe.
While sleep loss doesn’t necessarily affect every aspect of cognition, a detailed report published in 2009 in the Seminars in Neurology journal explained it has been found to have measurable effects on vigilance and attention, working (short-term) memory, multitasking, flexible thinking, risk assessment, insight and decision-making abilities. Even chronic partial-sleep loss of four to seven hours a night over several nights (not just total sleep deprivation) is associated with these side effects.
So, how much shut-eye do you really need?
Recently, the National Sleep Foundation conducted an in-depth review of sleep research to revise its guidelines. During this analysis, the panel of medical and sleep experts formally reviewed more than 300 articles published in peer-reviewed journals.