A new paper based on an analysis of sleep and cognitive (brain function) data from 3,968 men and 4,821 women who took part in the English Longitudinal Study of Ageing (ELSA) indicates that sleep problems are associated with worse memory and executive function in older people. 

Respondents reported on the quality and quantity of sleep over the period of a month and the results showed that there is an association between both quality and duration of sleep and brain function which changes with age.

In adults aged between 50 and 64 years of age, short sleep (<6 hours per night) and long sleep (>8 hours per night) were associated with lower brain function scores. By contrast, in older adults (65-89 years) lower brain function scores were only observed in long sleepers.

Cognitive function scores by sleep quantity. Fully adjusted mean T scores for amnestic and non-amnestic cognition scores for each sleep quantity category, in younger and older age groups. Adjusted for age, sex, sleep age, education, employment grade, depression, physical activity, smoking, general health. 
doi:10.1371/journal.pone.0100991

Dr. Michelle A Miller says, "6-8 hours of sleep per night is particularly important for optimum brain function, in younger adults. These results are consistent with our previous research, which showed that 6-8 hours of sleep per night was optimal for physical health, including lowest risk of developing obesity, hypertension, diabetes, heart disease and stroke."

Interestingly, in the younger pre-retirement aged adults, sleep quality did not have any significant association with brain function scores, whereas in the older adults (>65 years), there was a significant relationship between sleep quality and the observed scores.

"Sleep is important for good health and mental wellbeing," says Professor Francesco Cappuccio, "Optimizing sleep at an older age may help to delay the decline in brain function seen with age, or indeed may slow or prevent the rapid decline that leads to dementia."

Miller concludes that "if poor sleep is causative of future cognitive decline, non-pharmacological improvements in sleep may provide an alternative low-cost and more accessible Public Health intervention, to delay or slow the rate of cognitive decline."