LOS ANGELES, May 1 (Xinhua) -- Middle-aged adults who sleep too less or too much may be more likely to suffer cognitive decline, a new study suggests.
According to the study, less than six hours of sleep each night is considered too little and more than eight hours as too much for middle-aged adults.
The study, conducted by researchers at University College London Medical School, was published May 1 in the American medical journal Sleep.
The researchers conducted the study in two periods -- the 1997- 1999 period and the 2003-2004 period. 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.
The researchers compared those who reported changes in their sleep patterns with people whose sleep duration stayed the same over the course of the study.
In the follow-up, each individual was given a battery of standard tests to assess his or her memory, reasoning, vocabulary, global cognitive status and verbal fluency.
The study findings show that 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.
"Sleep provides the body with its daily need for physiological restitution and recovery," explained Jane Ferrie, a senior research fellow in the department of epidemiology and public health at the school. "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. Editor: yan