いつのまにか
before one knows, unnoticed
You did not see it get light and you were sitting there awake the whole time. It is two いつ, and then it is not, and you never saw the gap: いつのまにか.
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Emaki is not affiliated with, or endorsed by, Kaishi 1.5k or its authors. The word, reading and meaning come from the Kaishi 1.5k deck, used with their permission. The mnemonic, the note and the kanji breakdown are original work, published under CC BY-SA 4.0.