This study is designed to compare dementia (dementia as used here refers to a chronic, global deterioration of intellectual function and personality acquired in later life (i.e., over 50 years of age)) with normal aging in an effort to describe the nature of selected neurobiological factors that might underlie and/or indicate the onset and progression of the dementing process. Changes in intellectual function and in electrophysiological measures (the sleep and waking EEG) are known to be profound in patients with dementia. In fact, diurnal sleep patterns are often fragmented by long periods of wakefulness at night and by lengthy sleep episodes in the day. Sleep variables and particularly rapid eye movement (REM) sleep time, are known to correlate signigicantly with intellectual functioning among normal elderly and brain damaged subjects. It is hypothesized that other selected sleep variables (e.g., EEG slow wave activity) may also correlate well with mental function decline in aged demented patients. This study is designed to evaluate sleep pattern and mental function variables several times over the course of the disease in selected dementia patients of the Alzheimer's type (dementia of the Alzheimer's type refers to dementia occurring secondary to damaged or diffusely lost brain tissue, identifiable by specific neuropathologic signs of primary neuronal degeneration post mortem). These assessments of sleep function can then be compared with the patient's life expectancy and medical progress to determine whether selected sleep variables can be profitably used as an objective evaluation of the severity and stage of the dementing process.