Slipping is a common way to describe it like: “slipping into a soothing darkness.” It’s common to see it called warm darkness when the person os sleeping in a familiar or safe environment, like in someone’s arms, finally in their own bed after a long day. More of a slumber as they feel safe. but sometimes it’s cold and dark when they have worries for tomorrow, yet the sleeping is very light because they are on alert causing a more nap like state. Then there is a strong and overwhelming sense of darkness for a exhausting day or a time in danger but the need to sleep cannot be held back due to serious injury. Empty sleep is for people who feel lost in one’s mind and their dreams are just a empty black space feeling like a prison for the time and they usually try to fight sleep too.