夜更し
よふかし · yofukashi · JLPT N1 · noun
staying up late
夜 — 亠 lid + 亻 person + 夕 evening moon
夜 originally depicted a person (亻) under a roof (亠) with the moon (夕) beside them, evoking nighttime rest. The moon component 夕 carries the meaning of evening, while the surrounding elements set the scene of a person at night. Over time it came to mean 'night' in general.
更 — 丙 (bǐng) phonetic + 攴/攵 (action/strike)
Originally an oracle-bone graph showing a hand striking a watch-drum to mark night watches, hence 'to change shifts.' From this came the meanings 'to change' and 'to replace' (gēng). The reading gèng later developed the sense 'further, even more,' as in 'one watch further on.'