Sleep has typically been shrouded in mystery, and rapid eye movement sleep especially so, since that's when we dream. Researchers at Douglas Mental Health University Institute and McGillUniversity have identified how specific neural circuits cause REM sleep. Previous work had shown associations between these circuits and REM sleep, but researchers didn't know if this neural activity actually caused REM sleep or was a by-product of it. Using optogenetics, the researchers were able to induce REM sleep in mice and control how long it lasted by activating a specific neural circuit in the lateral hypothalamus. The results, published in Nature Neuroscience, are anticipated to bring a better understanding of sleep and sleep disorders in humans.