Sleep is essential for life. However, its precise biological function remains largely unknown. Our lab seeks to unravel the evolutionary origins and significance of sleep by combining molecular, cellular, and systems-level approaches. We study a diverse range of animal species including nematodes, fish, and mice. For example, the nematode Caenorhabditis elegans has only 302 neurons, yet we have uncovered evidence that its sleep-like state is evolutionarily conserved with mammalian sleep. In mice, we have identified specific neurons essential for controlling REM sleep—the stage associated with vivid dreaming—and successfully developed models that allow us to manipulate REM sleep and even exhibit dream-enacting movements during sleep. Ultimately, we aim to connect basic biological discoveries with clinical applications, contributing to the prevention and treatment of aging-related disorders such as dementia and psychiatric illness.