The phrase "in the arms of Morpheus" is often used as a euphemism for being asleep. However, there are several other expressions that can be used to convey the same meaning. One of the most commonly used synonyms is "in dreamland," which refers to the world of dreams. Another popular option is "in slumber," which emphasizes the deep and restful nature of sleep. "Asleep at the switch" is another related phrase that indicates a state of unconsciousness, but with a negative connotation, suggesting negligence or inattention. "Sound asleep" is yet another term that implies a deep, uninterrupted rest. Finally, "dead to the world" refers to being completely unaware of one's surroundings while asleep, and is often used to describe particularly deep sleep.