TOO little or too much sleep increases the rate of mental decline among older women and may contribute to Alzheimer's.
Researchers 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.
The women, participants in the Nurses Health Study in the US, had their mental functioning assessed every other year for six years from 70 onwards.
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.
Study leader Elizabeth Devore, from Brigham and Women's Hospital in Boston, said: "Our findings support the notion that extreme sleep durations and changes in sleep duration over time may contribute to early Alzheimer's changes in older adults.
"The public health implications of these findings could be substantial, as they might lead to the eventual identification of sleep -- and circadian [body clock] -- based strategies for reducing risk of cognitive impairment and Alzheimer's."
The findings were presented at the Alzheimer's Association International Conference in Vancouver.
Three other studies showed links between sleep disorders and abnormal 'body clock' patterns and mental impairment.
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 that the quality and duration of sleep are also linked to cognitive health.
"We're not saying you shouldn't enjoy the occasional lie-in, but good quality sleep, eating a balanced diet, maintaining a healthy weight and exercising regularly can all make a difference in reducing your risk."