夜不能寐

A four-character set phrase rather than a canonical chengyu — it isn't found in 《新华成语词典》 but functions exactly like one in modern writing. Always describes wakefulness driven by an emotion (worry, anxiety, anticipation, grief) — never a medical insomnia complaint. Compare with 辗转反侧 (zhǎnzhuǎn fǎncè, 'tossing and turning,' from 《诗经》) which emphasizes the physical restlessness.

夜 'night' + 不能 'unable to' + 寐 'to sleep' (the literary verb for sleep, also seen in 梦寐以求 'long for in one's dreams'). Word-for-word: 'at night, unable to sleep.'