night light
A small, dim light left on throughout the night, often in a hallway, child's room, or bathroom.
子供部屋に終夜灯をつけておく。
I keep a night light on in the children's room.
Compound of 終夜 (all night) and 灯 (light). The exact historical derivation is uncertain, but the spelling is conventionally associated with a light that stays on through the night.