The phrase "sleep soundly" is often used to describe a deep and restful sleep without interruption. There are several synonyms for this phrase that can be used in writing or conversation. One such synonym is "rest peacefully", which conveys a sense of calm and tranquility. Another option is "slumber soundly", which emphasizes the quality of the sleep rather than the act of sleeping itself. "Snooze deeply" is another synonym that describes a state of uninterrupted sleep. Other synonyms include "doze soundly", "rest deeply", and "sleep peacefully". Overall, using synonyms for "sleep soundly" helps to add variety to writing and avoid repetition.