Disclaimer: based on characters and situations owned by Toei Company, Saban Entertainment and Disney
Curiosity is what drove the Ozu-siblings to do what they were doing that night. They had one new person living under their roof, a necromancer, whom only spoke to them whenever he was spoken to. And even then, he usually made snide remarks on them, about their inexperience and the fact that he's so much stronger than they are. For reasons they had never known, that necromancer, Sutokeru, never did show himself during the day, unless they needed him for some reason. The only moment of the day they could see him, was when everyone else was still asleep.
With their parents gone for the night, the rest of the week even, the Ozu-siblings thought they should stay up for the night, trying to see what it is that keeps Sutokeru busy at night. Whether it is a plan to take down their currant enemy Lord Zedd, whether it's a way to revive all of the dead, perhaps even a grand scheme to destroy the realm of the living. Call it the family's need to know.
And so, they had spend the entire day preparing themselves for the night. When Sutokeru is awake...