I am familiar with the effects of sleep deprivation and some of the harmful effects of oversleeping. However, I have also heard more than once that sleeping more than 10 hours on average, even for a non-sedentary person, can lower overall brain alertness and acuity. In addition, I have heard that it can cause the premature onset of Alzheimer's disease and other brain-related diseases.
This is something entirely unfamiliar to me and I was wondering: Is this supported by scientific evidence?