Study objectives: Sleep disturbances are considered both a risk factor and symptom of dementia. The present research aimed to identify cognitive tests in which performance is associated with objective sleep quality or quantity, focusing on cognitive tests designed to evaluate the earliest cognitive changes in dementia. Methods: We recruited younger and older adults and remotely monitored their sleep patterns for 7 consecutive days using wrist actigraphy and sleep diaries. On day 7, participants completed a battery of cognitive tests, which included the Psychomotor Vigilance Task (PVT), the Prodromal Alzheimer's and Mild Cognitive Impairment battery from the Cambridge Neuropsychological Test Automated Battery (CANTAB), and the Mnemonic Similarity Task (MST), designed to tax pattern separation. The older adults were also assessed with the Montreal Cognitive Assessment (MoCA). Results: The final sample included 81 younger adults (mean age: 20.12, SD = 1.99) and 34 older adults (mean age: 65.56, SD = 9.57). There were no significant relationships between sleep and cognitive performance in the younger adults. In the older adults, there were significant correlations between total sleep time and PVT and MST performance. MoCA scores were correlated with performance on CANTAB and MST. Total sleep time also predicted MST performance in the older adults when controlling for age and gender. Conclusions: Performance on cognitive tests designed to assess pattern separation are sensitive to older adults sleep duration and the early cognitive changes associated with dementia. MST should be evaluated for potential use as a clinical trial outcome measure for sleep-promoting treatments in older adults.