Back in 2009, The University of Hull used functional magnetic resonance (fMRI) to study the brain activity of hypnotised patients and noticed decreased activity in areas of the brain that support the “default mode” network. These areas are accessed by the brain when resting and day dreaming.
The link can be found here: http://www2.hull.ac.uk/news-and-events/news-archive/2009-news-archive/november/brain-activity-changes-during.aspx