This spell helps you and your party sleep extra soundly. It doesn't give you the ability to put anyone into sleep, just helps people sleep better. The spell uses up the current selected ghost, or a ghost you target.
One of the more peaceful ways to use the ghosts, restful sleep asks a ghost to guard your party's sleep. The ghost is unable to fend off dangerous creatures and the like, but it is capable of handling non-mundane things like bad dreams, resulting in more restful sleep. This is once again a rather late addition to the guild, one which could have saved Samuel de Montfort's life, had he known this technique.
Usage: cast restful sleep cast restful sleep at <ghost name>