Historically, sleep has been seen as a passive state, and yet more recent research demonstrates that sleep is a time for the brain and body to actively fulfill functions pertaining to cognition, metabolism, and cellular maintenance. Clinical studies of sleep using polysomnography have demonstrated profound sleep disturbances in Down syndrome and Alzheimer disease. Deriving from these basic studies is a better understanding of the networks, neurotransmitters systems, and physiological and genetic factors that regulate the daily organization of sleep and wake cycles. Currently, physiological and pharmacological means for manipulating sleep are being developed. Therefore, it seems plausible that sleep and EEG studies may soon be used to develop new biomarkers for Down syndrome and Alzheimer’s disease and, in turn, improve quality of life.