熟睡 literally combines 熟 (shú), meaning 'thorough' or 'deep', and 睡 (shuì), meaning 'to sleep'. Together, they form a noun meaning 'deep sleep' — the state of sleeping soundly and without disturbance, often with slow breathing and minimal responsiveness to external stimuli. It emphasizes the quality and depth of sleep rather than just the act of sleeping.

This term is commonly used in health, medical, or daily routine contexts to describe restorative sleep — for example, when discussing sleep hygiene, children’s nighttime routines, or recovery from illness. It’s more formal and descriptive than the general word 睡觉 (shuìjiào, 'to sleep') and carries a connotation of peaceful, uninterrupted slumber. You’ll often see it in written Chinese (e.g., health articles, parenting guides) and in careful spoken descriptions, but rarely in casual conversation where simpler terms suffice.