The Sleep Card isn't harmful to the one affected by it's powers, although it's still a good idea to be cautious around this little pixie like being. It flies all over the place, spreading a magical dust which can almost instantly put anything to sleep, even magical beings like Kero!
In Episode 22, it attacked and placed Madison and other people under it's spell of slumber. Sakura and Kero gave chase, but even Kero ws not immune to Sleep's powers. Sakura finally managed to summon the Windy Card , allowing Sakura to trap and seal the pixie. When summoned, it will spread about the same magical dust which puts whoever is desired to sleep.
