TOO little or too much sleep increases the rate of mental decline among older women and may contribute to Alzheimer's, a study has shown.
Researchers in the US studied sleep duration in more than 15,000 women aged 70 and older.
Those who slept for five hours or less a night performed less well in mental tests than those who slept for seven hours.
But spending too long in bed was not good for the brain either. Nine hours or more of sleep also led to reduced mental performance. Sleeping less or more than seven hours had an effect equivalent to ageing by two years.
Average sleep duration was recorded twice, when the women were aged 40 to 65, and again at 54 to 79.
Women who changed the amount of time they slept by two hours or more from mid to later life suffered more mental decline than those whose sleep pattern stayed the same.
Dr Anne Corbett, research manager at the Alzheimer's Society, said: "A good night's sleep is one of life's pleasures but once again, this robust research suggests the quality and duration of sleep are also linked to our cognitive health."
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