The bride and groom were sleeping peacefully with smiles on their faces.

peacefully 是形容词 peaceful 的副词形式，peaceful 由名词 peace（和平、安宁）加上后缀 -ful（充满...的）构成，意思是"平静的、安宁的"。peacefully 表示"安详地、平静地"，描述一种宁静的状态。