Powernap is a command that allows you to quickly replenish your energy, at the cost of leaving you temporarily vulnerable to all the nasties in the world; because you will be asleep.
In addition, powernapping regularly increases you chance of experiencing a range of nightmarish dream sequences; during which you can develop your backstory, learn more about the world of Epitaph and decide on your Profession.
Powernap can be used on its own (i.e. "powernap") or in conjunction with an object in the room you are in (e.g. "powernap on table" or "powernap in bed").
Low resolve and/or low energy increase chance on getting nightmare. (Source: Elorf, four nightmares after eachother at extremely low resolve)