There are several synonyms that can be used to describe the act of lying down. One common synonym is "reclining." Reclining implies a relaxed and comfortable position where one is lying down, but not necessarily sleeping. Another synonym is "resting," which is often used to describe a state of relaxation or recovery. "Flat-out" can be used to describe the act of lying down in a flat or horizontal position, while "prostrate" suggests a position of complete submission or defeat. "Supine" is a medical term used to describe lying face up, while "prone" implies lying face down. Ultimately, the choice of synonym will depend on the context and the intended meaning.