also: ねごみ
noun
being asleep in bed; being sick in bed
Refers to the state of being in bed, either sleeping or ill. Often used in phrases like 寝込みを襲う (to attack someone while they are asleep) or 寝込みに (while someone is in bed).
泥棒は寝込みを襲って金を奪った。
The burglar attacked while the victim was asleep and stole the money.
彼は寝込みに電話をかけられて怒った。
He got angry because he was called while he was in bed.
寝込む is the verb meaning 'to stay in bed (due to illness)' or 'to be fast asleep'. 寝込み is the noun form derived from it.
Noun form of the verb 寝込む (ねこむ), from 寝 (ね, 'sleep') + 込む (こむ, 'to go into, to be deep in').