force of habit

(idiomatic) an act that has been repeated to the point where the performance of the act becomes automatic

Example:   "Only force of habit will explain why people today do honor to Christopher Columbus." - New York Times, 1937
1900, L. Frank Baum, The Wonderful Wizard of Oz
  The Tin Woodman lay down on his bed from force of habit, for he remembered when he was made of flesh; but not being able to sleep, he passed the night moving his joints up and down to make sure they kept in good working order.