眠 is a bound form (b.f.) — it cannot stand alone as a word. Instead, it appears as a morpheme in compounds related to sleep or dormancy, such as 睡眠 (sleep), 失眠 (insomnia), 安眠 (peaceful sleep / to euthanize), 催眠 (hypnotize), and 冬眠 (hibernation). The meaning covers both literal sleep and metaphorical dormancy (e.g., in animals and plants). As an advanced HSK 7 item, you'll encounter it mostly in formal or specialized vocabulary.