The brain never turns off, not even while sleeping. “Cells are always firing,” says William Dement, Ph.D., a professor of psychiatry and behavioral sciences at Stanford University, and founder of the world’s first sleep laboratory. While the mind can become idle during slumber, sometimes it can create a fabricated world that can feel like reality for most. Dement delves into what happens during REM and how some dreams can feel more intense than others.

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