The body never adjusts
A common misconception about sleep is that the circadian rhythm will eventually adjust, making it possible to greatly change your sleep schedule on the fly. You see this often when someone begins working night shifts and thinks that their body will adapt to sleeping during the day and being awake at night. Unfortunately for them, that is not how it works. Your body will never adjust to an entirely different sleep schedule, and your circadian rhythm will remain relatively stable throughout your life. In order to be successful in night shifts, you have to have always been more awake and alert at night.