A spokesman confirms Perry suffered "mild sleep apnea" during his presidential campaign.
One of the most memorable mistakes on the campaign trail came during a Michigan debate in which Perry couldn't remember all three federal agencies he had proposed to eliminate.
A new book written by a Texas newspaper reporter explains Perry's condition went undetected for years.
It appears to have been kept in check by exercise.
Back surgery in July 2011 derailed such physical activity, which the book asserts changed Perry from light sleeper to insomniac.
A spokesman with Perry's office says once he could again exercise regularly, the symptoms went away.