A special laboratory at the University of Chicago is busy only at night. It is a dream laboratory where researchers are at work studying dreamers. Their findings have discovered that everyone dreams from three to seven times a night, although in ordinary life a person may remember none or only one of his dreams. While the subjects--usually students--sleep, special machines record their brain waves and eye movements as well as the body movements that signal the end of a dream. Surprisingly, all subjects sleep soundly. Observers report that a person usually fidgets  before a dream. Once the dream has started, his body relaxes and his eyes become more active, as if the curtain had gone up on a show. As soon as the machine shows that the dream is over, a buzzer wakens the sleeper. He sits up, records his dream, and goes back to sleep--perhaps to dream some more. Researchers have found that if the dreamer is wakened immediately after his dream, he can usually recall the entire dream. If he is allowed to sleep even five more minutes, his memory of the dream will have disappeared. The machines being used in the experiment record   _  .
the depth of sleep the subjects' brain waves and eye movements how many dreams a person has what a sleeper dreams during his sleep