In experiments, researchers monitored three groups of participants who slept 4, 6, or 8 hours a night over an extended amount of time. After just two weeks, the group who slept 6 hours had a similar reaction time as a person whose blood alcohol concentration was 0.1 percent. Those who slept 4 hours would fall asleep during their cognitive tests.
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People who consistently sleep less than 7 to 8 hours a night may suffer cognitive issues. People who sleep more than 8 hours don’t have cognitive impairment, but like the chronically sleep deprived, they have higher risks of heart disease, obesity, and diabetes.
There also appears to be a genetic mutation that allows a person to sleep less because of their more intense sleep sessions.