The report was published in the May 1 issue of 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.
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 percent of men and 50 percent of women continued to sleep the same amount each night. However, 7.4 percent of women and 8.6 percent 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 percent of women and 18 percent of men reported decreases in their sleep -- dozing less than six, seven or eight hours per night.
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 v
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