Why is Renfield so volatile and inconsistent in Chapter 9??

Renfield is a madman in Seward's asylum. He is obsessed with consuming as much life as possible: he eats flies, spiders, and even birds while they are raw and live. In Dr. Seward's diary, we learn more about Renfield. Locked up, he continually murmurs to himself, "I can wait." He escapes again, and is found once more at the door of the chapel of Carfax. As the attendants try to subdue the enraged madman, Renfield grows suddenly calm at the sight of a giant bat flying across the sky.