godan verb (-ru), intransitive verb
Describes the moment of drifting off to sleep.
本を読んでいるうちに寝入ってしまった。
I fell asleep while reading a book.
godan verb (-ru), intransitive verb
sleep soundly
Implies a deep, peaceful sleep.
I was tired, so I slept soundly.
godan verb (-ru), intransitive verb
become dull; lose vigor; slacken
Rare figurative use; describes a decline in energy or sharpness, often in literary contexts.
長雨で気持ちが寝入ってしまった。
My spirits became dull from the long rain.