http://goo.gl/kQ3MvF
The study 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 (<6hrs per night) and long sleep (>8hrs 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.
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".