Having too much sleep in middle age could be as bad as not getting enough, a new research revealed.
A study of almost 9,000 men and women found that people aged 50 to 64 who sleep for less than six and more than eight hours a night have worse decision-making ability and memories.
But for older adults aged 65 to 89, brain power was only impaired if they slept over eight hours a night.
Experts at the University of Warwick found that getting the right amount of sleep at an older age may prevent mental illness in later life such as dementia.
Researcher Dr Michelle Miller said that the amount of sleep people need and its effect on their health changes with age.
"Six to eight hours of sleep per night is particularly important for optimum brain function in younger adults," Dr Miller said.
"These results are consistent with our previous research, which showed six to eight hours per night was optimal for physical health, including lowest risk of developing obesity, hypertension, diabetes, heart disease and stroke," she added.