To Freud, dreaming provides a playground for the unconscious mind; to Jung, it is a stage where the psyche’s archetypes act out primal themes. Recent theories hold that dreams help the brain to consolidate emotional memories and to work through current life problems.

Now, in a new paper published in the journal Natural Reviews Neuroscience, Dr. J. Allan Hobson, a psychiatrist and longtime sleep researcher at Harvard, argues that the main function of rapid-eye-movement sleep, or REM, when most dreaming occurs, is physiological.

The brain is warming its circuits, anticipating the sights and sounds and emotions of waking, tuning the mind for conscious awareness. “It’s like jogging; the body doesn’t remember every step, but it knows it has exercised. It has been tuned up,” says Hobson. “It’s the same idea here.” The theory might help explain why people forget so many dreams.

Hobson co-authored another paper with Ursula Voss, of J.W. Goethe-University in Frankfurt, in the journal Sleep, where scientists found that lucid dreaming, one of many examples of a mixed mental state, has elements of both REM and waking awareness. In a lucid dream, Hobson explains, “You are seeing the split brain in action. This tells me that there are these two systems, and that in fact, they can be running at the same time.”