As we usually think of dreams, they go on in the minds of people who are actually lying in a real bed in a real house, even if in the dream they are running away from a homicidal lawnmower through the streets of Kansas City. We also assume that normal dreams depend on what is happening in the dreamer's brain while he sleeps."
I don't know the meaning of the word "As" in the first sentence. Could you help me please! I think it means "when". Is it right?