 more or less than seven hours of sleep, more or less, was linked to reduced mental processing speed and problem-solving skills. The researchers say one reason is that getting less than seven hours a night means people are getting less deep sleep. Without that restorative deep sleep, damaging proteins build up in the brain, which is one of the hallmarks of dementia, by the way, sleeping more than eight hours a night after middle age is linked to depression. So to maximize your brain function, seven hours is your sweet spot. John Tesh with today's Intelligence for Your Life Minute.