From modern scholarly Ancient Greek ὕπνωσις (húpnōsis), which formation closes the derivational gap between the ancient words ὑπνόω (hupnóō, “I put to sleep”) and ὑπνωτικός (hupnōtikós, “sleep-inducing, soporific”). Ultimately traces back to ὕπνος (húpnos, “sleep”).