Why Flight Attendants Say Seat Swaps On Overnight Flights Are Actually A Safety Issue

For flight attendants, one of the common problems during boarding is when a passenger wants to move seats. It may be that they did not get their seat preference, or that their family has been split up or is not sitting with their companion. What can a passenger do, and what is the correct etiquette to use? Do you need permission to move, and can you ask another passenger to move from their seat to help you out?