En español | A good night's sleep isn't just refreshing: New research shows that lack of sleep ages the brain. Researchers asked study participants how much sleep they were getting — first as a baseline then five years later. Those whose sleep decreased during the five years showed an accelerated mental decline during cognitive testing that was equal to an extra four to seven years of aging.

See also: Seven signs of sleep apnea.

"Sleep regenerates neurons that enable the brain to function optimally," says lead author Jane Ferrie, Ph.D., senior researcher at University College London Medical School. Intriguingly, the study found that those whose sleep increased during the five years also exhibited cognitive decline, possibly due to depression, heart disease, or some other illness. So what's optimal? Six to eight hours — consistently.

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