睡眠 refers specifically to the biological process of sleeping—the natural, periodic state of rest characterized by reduced consciousness, decreased bodily movement, and lowered sensory activity. Unlike the verb 睡 (shuì), which means 'to sleep' and can be used in everyday speech (e.g., 我要去睡了), 睡眠 is a formal, abstract noun emphasizing sleep as a physiological function or health-related concept. Both characters contribute to this meaning: 睡 carries the core idea of resting with eyes closed, while 眠 reinforces it—originally meaning 'to close the eyes' or 'to rest,' and now almost exclusively used in compound words related to sleep.

This term appears frequently in medical, scientific, and wellness contexts—such as discussions about sleep hygiene, disorders (e.g., 失眠), or research on sleep cycles. It’s rarely used in casual conversation as a standalone verb; instead, it functions as a subject or object in sentences about health, behavior, or biology. You’ll often see it paired with measure words like '小时' (hours) or modifiers like '充足' (sufficient) or '质量' (quality).