For middle-aged adults, sleeping less than six or more than eight hours a night is associated with a decline in brain function, British researchers contend.
The magnitude of that mental decline is equal to being four to seven years older, the researchers said.
"There is an expectation in today's 24-hour-a-day society that people should be able to fit more into their lives," said study author Jane Ferrie, a senior research fellow in the department of epidemiology and public health at University College London Medical School.
"The whole work/life balance struggle is causing people to trade in precious sleeping time to ensure they complete everything they feel is expected of them. Our study suggests that this may have adverse effects on their cognitive function," she said.
In fact, women who slept seven hours per night had the highest score for every cognitive measure, followed by those who had six hours of sleep. For men, cognitive function was similar for those who reported sleeping six, seven or eight hours. However, less than six hours of sleep - or more than eight hours - were associated with lower scores, Ferrie said.
Sleep helps the body recover
Noting that many biological processes take place at night, Ferrie explained that "sleep provides the body with its daily need for physiological restitution and recovery. While seven hours a night appears to be optimal for the majority of human beings, many people can function perfectly well on regular sleep of less or more hours".
However, since most research has focused on the effects of sleep deprivation on biological systems, it is not yet fully understood why seven hours is optimal - or why long sleeping appears to be detrimental, Ferrie said.
"Chronic short sleep produces hormones and chemicals in the body which increase the risk of developing heart disease and strokes, and other conditions like high blood pressure and cholesterol, diabetes and obesity," she added.
The report was published in the journal Sleep.
Ferrie's team collected data on 5,431 men and women, aged 35 to 55 in 1985, who took part in a long-term look at London-based office staff known as the Whitehall II study.
How many hours should you sleep?
In 1997-1999, the participants were asked how many hours they slept on an average week night, and were asked the same question in 2003-2004 after an average 5.4 years of follow-up. Those who reported changes in their sleep patterns were then compared with people whose sleep duration stayed the same over the course of the study.
In 2003-2004, each individual was given a battery of standard tests to assess his or her memory, reasoning, vocabulary, global cognitive status and verbal fluency.
The researchers found that during the study, 58% of men and 50% of women continued to sleep the same amount each night. However, 7.4 % of women and 8.6% of men increased their slumber from seven to eight hours per night.
This change in sleep pattern was associated with lower scores on six tests of cognitive function, compared with people whose sleep time did not change, the researchers found.
Only scores on the test of short-term verbal memory were not affected by sleeping more, they noted.
In addition, some 25% of women and 18% of men reported decreases in their sleep - dozing less than six, seven or eight hours per night.
Not enough sleep bad for the heart
This change was associated with lower scores on three of the six cognitive tests, with lower scores on the reasoning, vocabulary and global cognitive status tests, the researchers said. Surprisingly, increasing sleep from six hours or less had no beneficial effect, they added.
Dr Alberto Ramos, co-director of the Health Sleep Medicine Program and an assistant professor of clinical neurology at the University of Miami Miller School of Medicine, said various studies have shown sleeping too little or too much increases the risk of dying, having a heart attack or stroke and other health problems.
"Getting enough sleep helps many brain functions," Ramos said. "It is restorative, it lets you concentrate better and process new information better and faster."
It is not clear why too much sleep may be unhealthy, Ramos said. However, he speculates, it may be a sign of other health problems.
Ramos added that to stay healthy, sleep is as important as eating well and being physically active.
"We have to think of sleep the same way as we think about diet and exercise," Ramos said. "If we want to have a healthy lifestyle we think of diet and exercise, but part of the equation is that good sleep should be part of having a healthy lifestyle for healthy aging."
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