The idea of the file is cause the listener to fall asleep.
This would be the equivalent to a powerful sleeping pill, the file should probably be made for looping.
Other things could be done with or added to it such as making the sleep more restful, direct your mind to resolve life issues while you sleep, give the person peaceful or pleasant dreams. But the main idea is cause the listener to get restful sleep.
Some safety catches could be that they are told to wake up once rested or a set alarm goes off or some other emergency i.e. house on fire.