Loss of slow wave sleep (SWS) is common in mild cognitive impairment (MCI) and Alzheimer's disease, and is thought to worsen thinking, memory and brain degeneration. Initial studies suggest that correction of sleep deterioration may slow the progression of brain damage in mild cognitive impairment, and might be able to stop the development of Alzheimer's disease. Transcranial alternating current stimulation (tACS) uses electrodes to deliver very small amounts of electricity through the brain, with direct effects on brain cell function. Transcranial electric stimulation has been demonstrated to enhance slow wave sleep and to improve memory when applied during sleep in healthy adults. The purpose of this research is to investigate tACS to modulate sleep, thinking/memory, mood, and quality of life among normal healthy adults, older adults, as well as individuals with MCI.