Mid-nighter is a term that refers to someone who stays up late until midnight or beyond. There are other synonyms for mid-nighter, and some of them include night owl, insomniac, nocturnal, and owl. Night owl refers to a person who stays up late and prefers to work during the evening or late night hours. Insomniac, on the other hand, is someone who finds it difficult to fall asleep or stay asleep through the night. Nocturnal refers to animals or activities that are active at night, while an owl is a bird that mostly hunts at night.