Compare 眠る (nemuru, “to sleep”), 眠り (nemuri, “sleep”), 眠い (nemui, “sleepy”), and 眠たい (nemutai, “sleepy”), all ultimately related to Classical Japanese 寝 (nu, “to sleep”, modern 寝る (neru)).
おねむ • (o-nemu) ←おねむ (onemu)?