Scientists, for the first time, have analyzed the activity of the human brain during dreaming. Using lucid dreamers ~ people who are aware of their dreaming state and are able to alter a dream's content ~ they determined that brain activity during a dream matches the activity of the brain during a state of wakefulness.
According to ScienceDaily.com:
Methods like functional magnetic resonance imaging (fMRI) have enabled scientists to visualize and identify the precise spatial location of brain activity during sleep. However, up to now, researchers have not been able to analyze specific brain activity associated with dream content, as measured brain activity can only be traced back to a specific dream if the precise temporal coincidence of the dream content and measurement is known.
Whether a person is dreaming is something that could only be reported by the individual himself.
Scientists from the Max Planck Institute of Psychiatry in Munich, the Charité hospital in Berlin and the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig asked lucid dreamers to become aware of their dream actions while sleeping and then repeat certain actions while awake.