The generic guideline around sleep has historically been to aim for 8 hours. Now a significant research recently covered in the NY Times Magazine backs up that data. Those who had eight hours of sleep hardly had any attention lapses and no cognitive declines over the 14 days of the study. Those in the four- and six-hour groups had P.V.T. results that declined steadily with almost each passing day. Though the four-hour subjects performed far worse, the six-hour group also consistently fell off-task. By the sixth day, 25 percent of the six-hour group was falling asleep at the computer. By the end of two weeks, the six-hour sleepers were as impaired as those who, in another Dinges study, had been sleep-deprived for 24 hours straight — the cognitive equivalent of being legally drunk. Are you thinking you’re fine with 7 hours? A separate study that looked at a seven-hour group, their response time slowed and continued to do so for three days, before stabilizing at lower levels than when they started. So like it or not, those of us getting by thinking 7 hours is adequate are not thinking as clearly as we could be.